Surface Coating Constraint Induced Anisotropic Swelling of Silicon in Si-Void@SiO x Nanowire Anode for Lithium-Ion Batteries

Synergistic Protecting-Etching Synthesis of Carbon Nanoboxes@Silicon for High-Capacity Lithium-Ion Battery

Silicon (Si) is considered as the most likely choice for the high-capacity lithium-ion batteries owing to its ultrahigh theoretical capacity (4200 mA h g-1) being over 10 times than that of traditional graphite anode materials (372 mA h g-1). However, its widespread application is limited by problems such as a large volume expansion and low electrical conductivity. Herein, we design a hollow nitrogen-doped carbon-coated silicon (Si@Co-HNC) composite in a water-based system via a synergistic protecting-etching strategy of tannic acid. The prepared Si@Co-HNC composite can effectively mitigate the volume change of silicon and improve the electrical conductivity. Moreover, the abundant voids inside the carbon layer and the porous carbon layer accelerate the transport of electrons and lithium ions, resulting in excellent electrochemical performance. The reversible discharge capacity of 1205 mA h g-1 can be retained after 120 cycles at a current density of 0.5 A g-1. In particular, the discharge capacity can be maintained at 1066 mA h g-1 after 300 cycles at a high current density of 1 A g-1. This study provides a new strategy for the design of Si-based anode materials with excellent electrical conductivity and structural stability.

In Situ Hydrogel Polymerization to Form a Flexible Polysaccharide Synergetic Binder Network for Stabilizing SiOx/C Anodes

Today, the commercial application of silicon oxides (SiOx, 1 < x < 2) in lithium-ion batteries (LIBs) still faces the challenge of rapid performance degradation. In this work, by integrating hydrothermal and physicomechanical processes, water-soluble locust bean gum (LBG) and xanthan gum (XG) are utilized to in situ form an LBG@XG binder network to improve the performance of SiOx/C anodes. As a synergy of LBG and XG polysaccharides in hydrogel polymerization, LBG@XG can tightly wrap around SiOx/C particles to prevent plate damage. The flexible SiOx/C anode with the LBG@XG binder exhibits capacity retentions of 74.1% and 76.4% after 1000 cycles at 0.5 A g-1 and 1 A g-1, respectively. The full battery capacity remains stable for 100 cycles at 1 C and the rate performance is excellent (103 mAh g-1 at 3 C). This LBG@XG is demonstrated to be highly electronegative and has a strong attraction to SiOx/C particles, thereby reducing the expansion and increasing the stability of the SiOx/C anodes when coupled with the flexible binder network. In addition to the promising LBG@XG binder, this work also provides a research idea for developing green water-based binders suitable for application in the SiOx/C anodes of LIBs.

Dual carbon and void space confined SiOx/C@void@Si/C yolk-shell nanospheres with high-rate performances and outstanding cyclability for lithium-ion batteries anodes

Silicon-based anode materials with high theoretical capacity have great challenges of enormous volume expansion and poor electronic conductivity. Herein, a novel dual carbon confined SiO_x/C@void@Si/C yolk-shell monodisperse nanosphere with void space have been fabricated through hydrothermal reaction, carbonization, and in-situ low-temperature aluminothermic reduction. Furthermore, the O/Si ratio and void space between SiO_x/C core and Si/C shell can be effectively tuned by the length of aluminothermic reduction time. The SiO_x/C core plays a role of maintaining the spherical structure and the void space can accommodate the volume expansion of Si. Moreover, the inner and outer carbons not only alleviate volume variation of SiO_x and Si but also enhance the electrical conductivity of composites. Benefiting from the synergy of the double carbon and void space, the optimized VSC-14 anode affords prominent cycle stability with reversible capacity of 1094 mAh g^-1 after 550 cycles at 200 mA g^-1. By pre-lithiation treatment, the VSC-14 achieves an initial Coulombic efficiency of 93.27% at 200 mA g^-1 and a reversible capacity of 348 mAh g^-1 at 5 A g^-1 after 4000 cycles. Furthermore, the pouch cell using VSC-14 anode and LiFePO₄ cathode delivers a reversible capacity of 138 mAh g^-1 at 0.2C. We hope this strategy can provide a scientific method to synthesis yolk-shell Si-based materials.

Recovery of porous silicon from waste crystalline silicon solar panels for high-performance lithium-ion battery anodes

A low-cost and easy-available silicon (Si) feedstock is of great significance for developing high-performance lithium-ion battery (LIB) anode materials. Herein, we employ waste crystalline Si solar panels as silicon raw materials, and transform micro-sized Si (m-Si) into porous Si (p-Si) by an alloying/dealloying approach in molten salt where Li⁺ was first reduced and simultaneously alloyed with m-Si to generate Li-Si alloy at the cathode. Subsequently, the as-prepared Li-Si alloy served as the anode in the same molten salt to release Li⁺ into the molten salt, resulting in the production of p-Si by taking advantage of the volume expansion/contraction effect. In the whole process, Li⁺ was shuttled between the electrodes in molten LiCl-KCl, without consuming Li salt. The obtained p-Si was applied as an anode in a half-type LIBs that delivered a capacity of 2427.7 mAh g^-1 at 1 A g^-1 after 200 cycles with a capacity retention rate of 91.5% (1383.3 mAh g^-1 after 500 cycles). Overall, this work offers a straightforward way to convent waste Si panels to high-performance Si anodes for LIBs, giving retired Si a second life and alleviating greenhouse gas emissions caused by Si production.

Green and Scalable Fabrication of Sandwich-like NG/SiOx/NG Homogenous Hybrids for Superior Lithium-Ion Batteries

SiOx is considered as a promising anode for next-generation Li-ions batteries (LIBs) due to its high theoretical capacity; however, mechanical damage originated from volumetric variation during cycles, low intrinsic conductivity, and the complicated or toxic fabrication approaches critically hampered its practical application. Herein, a green, inexpensive, and scalable strategy was employed to fabricate NG/SiOx/NG (N-doped reduced graphene oxide) homogenous hybrids via a freeze-drying combined thermal decomposition method. The stable sandwich structure provided open channels for ion diffusion and relieved the mechanical stress originated from volumetric variation. The homogenous hybrids guaranteed the uniform and agglomeration-free distribution of SiOx into conductive substrate, which efficiently improved the electric conductivity of the electrodes, favoring the fast electrochemical kinetics and further relieving the volumetric variation during lithiation/delithiation. N doping modulated the disproportionation reaction of SiOx into Si and created more defects for ion storage, resulting in a high specific capacity. Deservedly, the prepared electrode exhibited a high specific capacity of 545 mAh g-1 at 2 A g-1, a high areal capacity of 2.06 mAh cm-2 after 450 cycles at 1.5 mA cm-2 in half-cell and tolerable lithium storage performance in full-cell. The green, scalable synthesis strategy and prominent electrochemical performance made the NG/SiOx/NG electrode one of the most promising practicable anodes for LIBs.

Recent Advances in Silicon-Based Electrodes: From Fundamental Research toward Practical Applications

The increasing demand for higher-energy-density batteries driven by advancements in electric vehicles, hybrid electric vehicles, and portable electronic devices necessitates the development of alternative anode materials with a specific capacity beyond that of traditional graphite anodes. Here, the state-of-the-art developments made in the rational design of Si-based electrodes and their progression toward practical application are presented. First, a comprehensive overview of fundamental electrochemistry and selected critical challenges is given, including their large volume expansion, unstable solid electrolyte interface (SEI) growth, low initial Coulombic efficiency, low areal capacity, and safety issues. Second, the principles of potential solutions including nanoarchitectured construction, surface/interface engineering, novel binder and electrolyte design, and designing the whole electrode for stability are discussed in detail. Third, applications for Si-based anodes beyond LIBs are highlighted, specifically noting their promise in configurations of Li-S batteries and all-solid-state batteries. Fourth, the electrochemical reaction process, structural evolution, and degradation mechanisms are systematically investigated by advanced in situ and operando characterizations. Finally, the future trends and perspectives with an emphasis on commercialization of Si-based electrodes are provided. Si-based anode materials will be key in helping keep up with the demands for higher energy density in the coming decades.

One-Step Low-Temperature Molten Salt Synthesis of Two-Dimensional Si@SiOx@C Hybrids for High-Performance Lithium-Ion Batteries

Various strategies have been developed to mitigate the huge volume expansion of a silicon-based anode during the process of (de)lithiation and accelerate the transport rate of the ions/electrons for lithium-ion batteries (LIBs). Here, we report a one-step synthetic route through a low-temperature eutectic molten salt (LiCl-KCl, 352 °C) to fabricate two-dimensional (2D) silicon-carbon hybrids (Si@SiO_x@MpC), in which the silicon nanoparticles (SiNPs) with an ultrathin SiO_x layer are fully encapsulated by graphene-like carbon nanosheets derived from a low-cost mesophase pitch. The combination of an amorphous graphene-like carbon conductive matrix and a SiO_x protective layer strongly promotes the electrical conductivity, structure stability, and reaction kinetics of the SiNPs. Consequently, the optimized Si@SiO_x@MpC-2 anode delivers large reversible capacity (1239 mAh g^-1 at 1.0 A g^-1), superior rate performance (762 mAh g^-1 at 8 A g^-1), and long cycle life over 600 cycles (degradation rate of only 0.063% every cycle). When coupled with a homemade nano-LiFePO₄ cathode in a full cell, it exhibits a promising energy density of 193.5 Wh kg^-1 and decent cycling stability for 200 cycles at 1C. The methodology driven by salt melt synthesis paves a low-cost way toward simple fabrication and manipulation of silicon-carbon materials in liquid media.

Facile Fabrication of High-Performance Si/C Anode Materials via AlCl3-Assisted Magnesiothermic Reduction of Phenyl-Rich Polyhedral Silsesquioxanes

Si/C composites, combining the advantages of both carbon materials and Si materials, have been proposed as the promising material in lithium-ion storage. However, up to now, the most common fabrication methods of Si/C composites are too complicated for practical application. Here, we first use phenyl-substituted cagelike polyhedral silsesquioxane (T_n-Ph, n = 8, 12) as both carbon and silicon precursors to prepare the high-performance Si/C anode materials via a low-temperature and simple AlCl₃-assisted magnesiothermic reduction. AlCl₃ plays two roles in the reduction process, on the one hand, it acts as liquid medium to promote the reduction of siloxane core in such a mild condition (200 °C), and on the other hand, it act as catalyst for phenyl groups polycondensation into carbon materials, which makes the procedure of fabrication feasible and controllable. Impressively, T₁₂-Si/C exhibits an excellent lithium anodic performance with a reversible capacity of 1449.2 mA h g^-1 with a low volume expansion of 16.3% after 100 cycles. Such superior electrochemical performance makes the Si/C composites alternative anode materials for lithium-ion batteries.

Designing superior solid electrolyte interfaces on silicon anodes for high-performance lithium-ion batteries

The solid electrolyte interface (SEI) is a passivation layer formed on the surface of lithium-ion battery (LIB) anode materials produced by electrolyte decomposition. The quality of the SEI plays a critical role in the cyclability, rate capacity, irreversible capacity loss and safety of lithium-ion batteries (LIBs). The stability of the SEI is especially important for Si anodes which experience tremendous volume changes during cycling. Therefore, in this review we discuss the effect of the SEI on Si anodes. Firstly, the mechanism of formation, composition, and component properties of solid electrolyte interfaces (SEIs) are introduced, and the SEI of native-oxide-terminated Si is emphasized. Then the growth model and mechanical failure of SEIs are analyzed in detail, and the challenges facing SEIs of Si anodes are proposed. Moreover, we highlight several modification methods for SEIs on Si anodes, including electrolyte additives, surface-functionalization of Si, coating artificial SEIs or protective layers, and the structural design of Si-based composites. We believe that designing a high-quality SEI is of great significance and is beneficial for the improved electrochemical performance of Si anodes.

Silicon Carbide as a Protective Layer to Stabilize Si-Based Anodes by Inhibiting Chemical Reactions

Developing a practical silicon-based (Si-based) anode is a precondition for high-performance lithium-ion batteries. However, the chemical reactivity of the Si renders it liable to be consumed, which must be completely understood for it to be used in practical battery systems. Here, a fresh and fundamental mechanism is proposed for the rapid failure of Si-based materials. Silicon can chemically react with lithium hexafluorophosphate (LiPF₆) to constantly generate lithium hexafluorosilicate (Li₂SiF₆) aggregates during cycling. In addition, nanocarbon coated on silicon acts as a catalyst to accelerate such detrimental reactions. By taking advantage of the high strength and toughness of silicon carbide (SiC), a SiC layer is introduced between the inner silicon and outer carbon layers to inhibit the formation of Li₂SiF₆. The side reaction rate decreases significantly due to the increase in the activation energy of the reaction. Si@SiC@C maintains a specific capacity of 980 mAh g^-1 at a current density of 1 A g^-1 after 800 cycles with an initial Coulombic efficiency over 88.5%. This study will contribute to improved design of Si-based anode for high-performance Li-ion batteries.

Nitrogen Plasma-Treated Core-Bishell Si@SiOx@TiO2-δ: Nanoparticles with Significantly Improved Lithium Storage Performance

Si-based anode materials have attracted considerable attention because of their ultrahigh reversible capacity. However, poor cycling stability caused by the large volume change during cycling prevented the commercial application of Si anodes for lithium-ion batteries (LIBs). To overcome these challenges, in the present study, we designed a nitrogen plasma-treated core-bishell nanostructure where the Si nanoparticle was encapsulated into a SiO_x shell and N-doped TiO_2-δ shell. Here, the SiO_x inside the shell and the TiO₂ outside the shell act as binary buffer matrices to accommodate the large volume change and also help to stabilize the solid electrolyte interphase films on the shell surface. More importantly, the plasma-induced N-doped TiO_2-δ shell with many Ti³⁺ species and oxygen vacancies plays a key role in improving the electrical conductivity of Si anodes. Owing to the synergistic effects of SiO_x and N-doped TiO_2-δ bishells, the cycling stability and rate performance of Si anodes are significantly enhanced. The as-obtained sample exhibits superior cycling stability with a capacity retention of 650 mA h g^-1 at 200 mA g^-1 after 300 cycles. This strategy is favorable for improving the electrochemical performances of Si-based anodes to employ in practical LIBs.

Prelithiated Surface Oxide Layer Enabled High-Performance Si Anode for Lithium Storage

SiO _x coating is an effective strategy to prolong the cycling stability of Si-based anodes due to the robust interaction between Si and the SiO _x layer. However, the SiO _x layer-protected Si anode is limited by the relatively low initial Coulombic efficiency and sluggish Li⁺ diffusion ability induced by the SiO _x layer. Herein, we present the preparation of selectively prelithiated Si@SiO _x (Si@Li₂SiO₃) anode by using a facile strategy to resolve the above issues. As the anode for lithium ion batteries, Si@Li₂SiO₃ exhibits a high initial Coulombic efficiency (ICE) of 89.1%, an excellent rate performance (959 mA h g^-1 at 30 A g^-1), and a superior capacity retention (3215 mA h g^-1). The full cell with LiFePO₄ cathode and Si@Li₂SiO₃ anodes is successfully assembled, disclosing a high ICE of 91.1% and excellent long cycling stability. The superior electrochemical performance of Si@Li₂SiO₃ can be attributed to the coating layer, which can strengthen the integrity of the electrode, decrease irreversible reactions, and provide efficient Li⁺ diffusion channels.

Improved Battery Performance of Nanocrystalline Si Anodes Utilized by Radio Frequency (RF) Sputtered Multifunctional Amorphous Si Coating Layers

Despite the high theoretical specific capacity of Si, commercial Li-ion batteries (LIBs) based on Si are still not feasible because of unsatisfactory cycling stability. Herein, amorphous Si (a-Si)-coated nanocrystalline Si (nc-Si) formed by versatile radio frequency (RF) sputtering systems is proposed as a promising anode material for LIBs. Compared to uncoated nc-Si (retention of 0.6% and Coulombic efficiency (CE) of 79.7%), the a-Si-coated nc-Si (nc-Si@a-Si) anodes show greatly improved cycling retention (C_50th/C_first) of ∼50% and a first CE of 86.6%. From the ex situ investigation with electrochemical impedance spectroscopy (EIS) and cracked morphology during cycling, the a-Si layer was found to be highly effective at protecting the surface of the nc-Si from the formation of solid-state electrolyte interphases (SEI) and to dissipate the mechanical stress upon de/lithiation due to the high fracture toughness.

Fabrication of double core–shell Si-based anode materials with nanostructure for lithium-ion battery

Yolk–shell structure is considered to be a well-designed structure of silicon-based anode. However, there is only one point (point-to-point contact) in the contact region between the silicon core and the shell in this structure, which severely limits the ion transport ability of the electrode. In order to solve this problem, it is important that the core and shell of the core–shell structure are closely linked (face-to-face contact), which ensures good ion diffusion ability. Herein, a double core–shell nanostructure (Si@C@SiO₂) was designed for the first time to improve the cycling performance of the electrode by utilising the unique advantages of the SiO₂ layer and the closely contacted carbon layer. The improved cycling performance was evidenced by comparing the cycling properties of similar yolk–shell structures (Si@void@SiO₂) with equal size of the intermediate shell. Based on the comparison and analysis of the experimental data, Si@C@SiO₂ had more stable cycling performance and exceeded that of Si@void@SiO₂ after the 276^th cycle. More interestingly, the electron/ion transport ability of electrode was further improved by combination of Si@C@SiO₂ with reduced graphene oxide (RGO). Clearly, at a current density of 500 mA g⁻¹, the reversible capacity was 753.8 mA h g⁻¹ after 500 cycles, which was 91% of the specific capacity of the first cycle at this current density.

Double core shell structure can not only improve the efficiency of lithium transport, but also effectively alleviate volume expansion.