Facile synthesis of core-shell structured Si@graphene balls as a high-performance anode for lithium-ion batteries

Core-Shell Semiconductor-Graphene Nanoarchitectures for Efficient Photocatalysis: State of the Art and Perspectives

Semiconductor photocatalysis holds great promise for renewable energy generation and environment remediation, but generally suffers from the serious drawbacks on light absorption, charge generation and transport, and structural stability that limit the performance. The core-shell semiconductor-graphene (CSSG) nanoarchitectures may address these issues due to their unique structures with exceptional physical and chemical properties. This review explores recent advances of the CSSG nanoarchitectures in the photocatalytic performance. It starts with the classification of the CSSG nanoarchitectures by the dimensionality. Then, the construction methods under internal and external driving forces were introduced and compared with each other. Afterward, the physicochemical properties and photocatalytic applications of these nanoarchitectures were discussed, with a focus on their role in photocatalysis. It ends with a summary and some perspectives on future development of the CSSG nanoarchitectures toward highly efficient photocatalysts with extensive application. By harnessing the synergistic capabilities of the CSSG architectures, we aim to address pressing environmental and energy challenges and drive scientific progress in these fields.

Tailoring the Size of Reduced Graphene Oxide Sheets to Fabricate Silicon Composite Anodes for Lithium-Ion Batteries

The integration of a silicon (Si) anode into lithium-ion batteries (LIBs) holds great promise for energy storage, but challenges arise from unstable electrochemical reactions and volume changes during cycling. This study investigates the influence of reduced graphene oxide (rGO) size on the performance of rGO-protected Si composite (Si@rGO) anodes. Two sizes of graphene oxide (GO(L) and GO(S)) are used to synthesize Si@rGO composites with a core-shell structure by spray drying and thermal reduction. Electrochemical evaluations show the advantages of the Si@rGO(S) anode with improved cycle life and cycling efficiency over Si@rGO(L) and pure Si. The Si@rGO(S) anode facilitates the formation of a stable LiF-rich solid electrolyte interface (SEI) after cycling, ensuring enhanced capacity retention and swelling control. Rate capability tests also demonstrate the superior high-power performance of Si@rGO(S) with low and stable resistances in Si@rGO(S) over extended cycles. This study provides critical insights into the tailoring of graphene-protected Si composites, highlighting the critical role of rGO size in shaping structural and electrochemical properties. The Si material wrapped by graphene with an optimal lateral size of graphene emerges as a promising candidate for high-performance LIB anodes, thereby advancing electrochemical energy storage technologies.

Innovative Solutions for High-Performance Silicon Anodes in Lithium-Ion Batteries: Overcoming Challenges and Real-World Applications

Silicon (Si) has emerged as a potent anode material for lithium-ion batteries (LIBs), but faces challenges like low electrical conductivity and significant volume changes during lithiation/delithiation, leading to material pulverization and capacity degradation. Recent research on nanostructured Si aims to mitigate volume expansion and enhance electrochemical performance, yet still grapples with issues like pulverization, unstable solid electrolyte interface (SEI) growth, and interparticle resistance. This review delves into innovative strategies for optimizing Si anodes' electrochemical performance via structural engineering, focusing on the synthesis of Si/C composites, engineering multidimensional nanostructures, and applying non-carbonaceous coatings. Forming a stable SEI is vital to prevent electrolyte decomposition and enhance Li+ transport, thereby stabilizing the Si anode interface and boosting cycling Coulombic efficiency. We also examine groundbreaking advancements such as self-healing polymers and advanced prelithiation methods to improve initial Coulombic efficiency and combat capacity loss. Our review uniquely provides a detailed examination of these strategies in real-world applications, moving beyond theoretical discussions. It offers a critical analysis of these approaches in terms of performance enhancement, scalability, and commercial feasibility. In conclusion, this review presents a comprehensive view and a forward-looking perspective on designing robust, high-performance Si-based anodes the next generation of LIBs.

Ag nanoparticle modified porous Si microspheres as high-performance anodes for Li-ion batteries

This study aimed to address the challenges associated with silicon (Si) anode materials in Li-ion batteries, such as their large volume effect and poor electrical conductivity. To overcome these limitations, a novel composite microsphere called pSi/Ag was developed using quartz waste through a combination of high-energy ball-milling, spray drying, and magnesiothermic reduction techniques. The morphology and structure of the pSi/Ag composite were thoroughly characterized using various methods, including X-ray diffraction, field-emission scanning electron microscopy, and transmission electron microscopy. The results revealed that the Ag nanoparticles were uniformly dispersed within the porous micron-sized Si sphere particles, leading to enhanced electrochemical performance compared to pure porous silicon that did not undergo the spray drying process. The use of micron-sized Si prevented the excessive formation of the solid electrolyte interphase film, and the pSi/Ag-5 anode, prepared with 5 wt% AgNO3 as a precursor, demonstrated an impressive initial Coulombic efficiency of 92.8%. Moreover, a high specific capacity of 1251.4 mA h g-1 over 300 cycles at a current density of 4000 mA g-1 was attributed to the improved conductivity provided by the Ag nanoparticles in the Si matrix. The straightforward synthesis method employed in this study to produce pSi/Ag presents a promising approach for the future development of high-performance silicon anodes in Li-ion batteries.

TiO2-LiF composite coating for improving NCM622 cathode cycling stability: one-step construction

The Ni-rich NCM622 is a promising cathode material for future high energy lithium ion batteries, but unstable electrochemical performance of NCM622 hinder its large scale commercial application. The cycling peformance of nickel-rich LiNi0.6Co0.2Mn0.2O2 (NCM622) cathode materials can be improved by surface coating. Here, a one-step approach based on TiF4 is used to successfully manufacture modified NCM622 cathode materials with a TiO2-LiF coating. The TiO2-LiF coated NCM622 preserves 79.7% capacity retention which is higher than the pure NCM622 (68.9%) at 1C after 200 cycles within 2.7-4.3 V. This material serves as the cathode for lithium-ion batteries (LIBs). The uniform TiO2-LiF coating layer can alleviate structural degradation brought on by unfavorable side reactions with the electrolyte has been validated. TiO2-LiF coated on NCM622 cathode materials can be modified easily by one-step approach.

Scalable engineering of hierarchical layered micro-sized silicon/graphene hybrids via direct foaming for lithium storage

Low-cost micro-sized silicon is an attractive replacement for commercial graphite anodes in advanced lithium-ion batteries (LIBs) but suffers from particle fracture during cycling. Hybridizing micro-sized silicon with conductive carbon materials, especially graphene, is a practical approach to overcome the volume change issue. However, micro-sized silicon/graphene anodes prepared via the conventional technique encounter sluggish Li+ transport due to the lack of efficient electrolyte diffusion channels. Here, we present a facile and scalable method to establish efficient Li+ transport channels through direct foaming from the laminated graphene oxide/micro-sized silicon membrane followed by annealing. The conductive graphene layers and the Li+ transport channels endow the composite material with excellent electronic and ionic conductivity. Moreover, the interconnected graphene layers provide a robust framework for micro-sized silicon particles, allowing them to transform decently in the graphene layer space. Consequently, the prepared hybrid material, namely foamed graphene/micro-sized Si (f-G-Si), can work as a binder-free and free-standing anode without additives and deliver remarkable electrochemical performance. Compared with the control samples, micro-sized silicon wrapped by laminated graphene layers (G-Si) and commercial micro-sized Si, f-G-Si maximizes the utilization of silicon and demonstrates superior performance, disclosing the role of Li+ diffusion channels. This study sheds light on the rational design and manufacture of silicon anodes and beyond.

Carbon Nanotube-Reinforced Dual Carbon Stress-Buffering for Highly Stable Silicon Anode Material in Lithium-Ion Battery

Silicon (Si) anode suffers from huge volume expansion which causes poor structural stability in terms of electrode material, solid electrolyte interface, and electrode, limiting its practical application in high-energy-density lithium-ion batteries. Rationally designing architectures to optimize the stress distribution of Si/carbon (Si/C) composites has been proven to be effective in enhancing their structural stability and cycling stability, but this remains a big challenge. Here, metal-organic frameworks (ZIF-67)-derived carbon nanotube-reinforced carbon framework is employed as an outer protective layer to encapsulate the inner carbon-coated Si nanoparticles (Si@C@CNTs), which features dual carbon stress-buffering to enhance the structural stability of Si/C composite and prolong their cycling lifetime. Finite element simulation proves the structural advantage of dual carbon stress-buffering through significantly relieving stress concentration when Si lithiation. The outer carbon framework also accelerates the charge transfer efficiency during charging/discharging by the improvement of lithium-ion diffusion and electron transport. As a result, the Si@C@CNTs electrode exhibits excellent long-term lifetime and good rate capability, showing a specific capacity of 680 mAh g^-1 even at a high rate of 1 A g^-1 after 1000 cycles. This work provides insight into the design of robust architectures for Si/C composites by stress optimization.

Modified preparation of Si@C@TiO2 porous microspheres as anodes for high-performance lithium-ion batteries

Microscale porous silicon materials have shown great application potential as anodes for next-generation lithium-ion batteries (LIBs); however, they face significant challenges, including mechanical structure instability, low intrinsic conductivity, and uncontrollable processing. In this study, a modified etching strategy combined with a facile sol-gel method is demonstrated to prepare microscale porous Si microspheres encapsulated by an inner amorphous carbon shell (≈10 nm) and an outer rigid anatase titanium oxide (TiO₂) shell (≈20 nm) (PSi@C@TiO₂), with the intact porous framework and core-shell-shell spherical structure. The interconnected pores can sufficiently accommodate the expansion of the Si core during lithiation. Moreover, the double shells can not only enhance the kinetic behavior of the PSi@C@TiO₂ microspheres, but can act as a compact fence to force the Si core to expand toward the internal pores during lithiation, ensuring the integrity of the porous spherical structure. As a result, the PSi@C@TiO₂ anodes show greatly superior high specific capacity, excellent rate capability, stable solid-electrolyte interphase (SEI) films and steady mechanical structure. It delivers a high reversible capacity of 1004 mA h g^-1 after 250 cycles at 0.5 A g^-1. This study provides a modified method to prepare microscale porous Si anodes with a stable mechanical structure and long cycle life for LIBs.

Encapsulating a Responsive Hydrogel Core for Void Space Modulation in High-Stability Graphene-Wrapped Silicon Anodes

Due to its formidably high theoretical capacity (3590 mAh/g at room temperature), silicon (Si) is expected to replace graphite as the dominant anode for higher energy density lithium (Li)-ion batteries. However, stability issues stemming from silicon's significant volume expansion (∼300%) upon lithiation have slowed down commercialization. Herein, we report the design of a scalable process to engineer core-shell structures capable of buffering this volume expansion, which utilize a core made up of a poly(ethylene oxide)-carboxymethyl cellulose hydrogel and silicon protected by a crumpled graphene shell. The volume expansion of the hydrogel upon exposure to water creates a void space between the Si-Si and Si-rGO interfaces within the core when the gel dries. Unlike sacrificial spacers, the dehydrated hydrogel remains in the core and acts as an elastic Li-ion conductor, which improves the stability and high rate performance. The optimized composite electrodes retain ∼81.7% of their initial capacity (1055 mAh/(g_rGO+gel+Si)) after 320 cycles when an active material loading of 1 mg/cm² is used. At more practical mass loadings (2.5 mg/cm²), the electrodes achieve 2.04 mAh/cm² and retain 79% of this capacity after 200 cycles against a lithium half-cell. Full cells assembled using a lithium ion phosphate cathode lose only 6.7% of their initial capacity over 100 cycles, demonstrating the potential of this nanocomposite anode for use in next-generation Li-ion batteries.

Complementary combinative strategy of defect engineering and graphene coupling for efficient energy-functional materials

The synergetic combination of defect engineering and graphene coupling enables to develop an effective way of exploring efficient bifunctional electrocatalyst/electrode materials. Defect-engineered amorphous MoO₂ -reduced graphene oxide (rGO) nanohybrid was synthesized by soft-chemical reduction of K₂ MoO₄ in graphene oxide colloids. Mo K-edge X-ray absorption spectroscopy clearly demonstrates the rutile-type local atomic structure of amorphous MoO₂ with significant oxygen vacancies and intimate electronic coupling with rGO. The defect-introduced MoO₂ -rGO nanohybrid shows excellent bifunctionality as electrocatalyst for hydrogen evolution reaction and electrode for sodium-ion batteries, which are superior to those of crystalline MoO₂ -rGO homologue. The beneficial effect of simultaneous defect control and rGO coupling can be ascribed to the provision of oxygen vacancies acting as active sites, the increase of electrical conductivity, and the improvement of reaction kinetics.

Surface Modification and Functional Structure Space Design to Improve the Cycle Stability of Silicon Based Materials as Anode of Lithium Ion Batteries

Silicon anode is considered as one of the candidates for graphite replacement due to its highest known theoretical capacity and abundant reserve on earth. However, poor cycling stability resulted from the “volume effect” in the continuous charge-discharge processes become the biggest barrier limiting silicon anodes development. To avoid the resultant damage to the silicon structure, some achievements have been made through constructing the structured space and pore design, and the cycling stability of the silicon anode has been improved. Here, progresses on designing nanostructured materials, constructing buffered spaces, and modifying surfaces/interfaces are mainly discussed and commented from spatial structure and pore generation for volumetric stress alleviation, ions transport, and electrons transfer improvement to screen out the most effective optimization strategies for development of silicon based anode materials with good property.

Controlling Void Space in Crumpled Graphene-Encapsulated Silicon Anodes using Sacrificial Polystyrene Nanoparticles

Silicon anodes have a theoretical capacity of 3590 mAh g^-1 (for Li₁₅ Si₄ , at room temperature), which is tenfold higher than the graphite anodes used in current Li-ion batteries. This, and silicon's natural abundance, makes it one of the most promising materials for next-generation batteries. Encapsulating silicon nanoparticles (Si NPs) in a crumpled graphene shell by spray drying or spray pyrolysis are promising and scalable methods to produce core-shell structures, which buffer the extreme volume change (>300 vol %) caused by (de)lithiaton of silicon. However, capillary forces cause the graphene-based materials to tightly wrap around Si NP clusters, and there is little control over the void space required to further improve cycle life. Herein, a simple strategy is developed to engineer void-space within the core by incorporating varying amounts of similarly sized polystyrene (PS) nanospheres in the spray drier feed mixture. The PS completely decomposes during thermal reduction of the graphene oxide shell and results in Si cores of varying porosity. The best performance is achieved at a 1 : 1 ratio (PS/Si), leading to high capacities of 1638, 1468, and 1179 mAh g^-1 _Si+rGO at 0.1, 1, and 4 A g^-1 , respectively. Moreover, at 1 A g^-1 , the capacity retention is 80.6 % after 200 cycles. At a practical active material loading of 2.4 mg cm^-2 , the electrodes achieve an areal capacity of 2.26 mAh cm^-2 at 1 A g^-1 .

Enhanced lithium storage performance of porous Si/C composite anodes using a recrystallized NaCl template

Silicon (Si) has recently aroused great interest as a promising anode material for lithium-ion batteries with high energy density due to its high theoretical capacity. However, the application of Si remains a great challenge owing to its extremely large volume change during cycling, thus resulting in dramatic capacity fading. Herein, a novel structure design of the porous Si/C composite with Si nanoparticles embedded in the carbon nanosheets has been successfully achieved by using a recrystallized NaCl template with appropriate particle size. The outermost sheet-like carbon coating can improve the electronic conductivity and contribute to the formation of a more stable solid-electrolyte interphase layer, while the inner void space effectively buffers the volume expansion of Si during the lithiation process. In addition, only a structure with Si particles anchored on the surface of carbon nanosheets has been obtained by using a commercial NaCl template with large particle size, confirming the effective regulation of the NaCl template in the microstructure and thus the electrochemical properties of the Si/C composites. As expected, benefiting from the combination of the outermost carbon coating and recrystallized NaCl-derived porous structure, the as-obtained Si/C composite demonstrates attractive cycling stability and rate performance as an anode material for lithium-ion batteries.