Micrometer-Sized SiMgy Ox with Stable Internal Structure Evolution for High-Performance Li-Ion Battery Anodes

What Is the Real Origin of Single-Walled Carbon Nanotubes for the Performance Enhancement of Si-Based Anodes?

A large amount of lithium-ion storage in Si-based anodes promises high energy density yet also results in large volume expansion, causing impaired cyclability and conductivity. Instead of restricting pulverization of Si-based particles, herein, we disclose that single-walled carbon nanotubes (SWNTs) can take advantage of volume expansion and induce interfacial reactions that stabilize the pulverized Si-based clusters in situ. Operando Raman spectroscopy and density functional theory calculations reveal that the volume expansion by the lithiation of Si-based particles generates ∼14% tensile strains in SWNTs, which, in turn, strengthens the chemical interaction between Li and C. This chemomechanical coupling effect facilitates the transformation of sp2-C at the defect of SWNTs to Li-C bonds with sp3 hybridization, which also initiates the formation of new Si-C chemical bonds at the interface. Along with this process, SWNTs can also induce in situ reconstruction of the 3D architecture of the anode, forming mechanically strengthened networks with high electrical and ionic conductivities. As such, with the addition of only 1 wt % of SWNTs, graphite/SiOx composite anodes can deliver practical performance well surpassing that of commercial graphite anodes. These findings enrich our understanding of strain-induced interfacial reactions, providing a general principle for mitigating the degradation of alloying or conversion-reaction-based electrodes.

Reduced Volume Expansion of Micron-Sized SiOx via Closed-Nanopore Structure Constructed by Mg-Induced Elemental Segregation

The inherently huge volume expansion during Li uptake has hindered the use of Si-based anodes in high-energy lithium-ion batteries. While some pore-forming and nano-architecting strategies show promises to effectively buffer the volume change, other parameters essential for practical electrode fabrication, such as compaction density, are often compromised. Here we propose a new in situ Mg doping strategy to form closed-nanopore structure into a micron-sized SiOx particle at a high bulk density. The doped Mg atoms promote the segregation of O, so that high-density magnesium silicates form to generate closed nanopores. By altering the mass content of Mg dopant, the average radii (ranged from 5.4 to 9.7 nm) and porosities (ranged from 1.4 % to 15.9 %) of the closed pores are precisely adjustable, which accounts for volume expansion of SiOx from 77.8 % to 22.2 % at the minimum. Benefited from the small volume variation, the Mg-doped micron-SiOx anode demonstrates improved Li storage performance towards realization of a 700-(dis)charge-cycle, 11-Ah-pouch-type cell at a capacity retention of >80 %. This work offers insights into reasonable design of the internal structure of micron-sized SiOx and other materials that undergo conversion or alloying reactions with drastic volume change, to enable high-energy batteries with stable electrochemistry.

Precursor Induced Assembly of Si Nanoparticles Encapsulated in Graphene/Carbon Matrices and the Influence of Al2O3 Coating on their Properties as Anode for Lithium-Ion Batteries

The theoretical capacity of pristine silicon as anodes for lithium-ion batteries (LIBs) can reach up to 4200 mAh g-1, however, the low electrical conductivity and the huge volume expansion limit their practical application. To address this challenge, a precursor strategy has been explored to induce the curling of graphene oxide (GO) flakes and the enclosing of Si nanoparticles by selecting protonated chitosan as both assembly inducer and carbon precursor. The Si nanoparticles are dispersed first in a slurry of GO by ball milling, then the resulting dispersion is dried by a spray drying process to achieve instantaneous solution evaporation and compact encapsulation of silicon particles with GO. An Al2O3 layer is constructed on the surface of Si@rGO@C-SD composites by the atomic layer deposition method to modify the solid electrolyte interface. This strategy enhances obviously the electrochemical performance of the Si as anode for LIBs, including excellent long-cycle stability of 930 mAh g-1 after 1000 cycles at 1000 mA g-1, satisfied initial Coulomb efficiency of 76.7%, and high rate ability of 806 mAh g-1 at 5000 mA g-1. This work shows a potential solution to the shortcomings of Si-based anodes and provides meaningful insights for constructing high-energy anodes for LIBs.

A Gradient Composite Structure Enables a Stable Microsized Silicon Suboxide-Based Anode for a High-Performance Lithium-Ion Battery

The practical application of microsized anodes is hindered by severe volume changes and fast capacity fading. Herein, we propose a gradient composite strategy and fabricate a silicon suboxide-based composite anode (d-SiO@SiOx/C@C) consisting of a disproportionated microsized SiO inner core, a homogeneous composite SiOx/C interlayer (x ≈ 1.5), and a highly graphitized carbon outer layer. The robust SiOx/C interlayer can realize a gradient abatement of stress and simultaneously connect the inner SiO core and carbon outer layer through covalent bonds. As a result, d-SiO@SiOx/C@C delivers a specific capacity of 1023 mAh/g after 300 cycles at 1 A/g with a retention of >90% and an average Coulombic efficiency of >99.7%. A full cell assembled with a LiNi0.8Co0.15Al0.05O2 cathode displays a remarkable specific energy density of 569 Wh/kg based on total active materials as well as excellent cycling stability. Our strategy provides a promising alternative for designing structurally and electrochemically stable microsized anodes with high capacity.

High-Entropy Engineering of Cubic SiP with Metallic Conductivity for Fast and Durable Li-Ion Batteries

A cost-effective, scalable ball milling process is employed to synthesize the InGeSiP3 compound with a cubic ZnS structure, aiming to address the sluggish reaction kinetics of Si-based anodes for Lithium-ion batteries. Experimental measurements and first-principles calculations confirm that the synthesized InGeSiP3 exhibits significantly higher electronic conductivity, larger Li-ion diffusivity, and greater tolerance to volume change than its parent phases InGe (or Si)P2 or In (or Ge, or Si)P. These improvements stem from its elevated configurational entropy. Multiple characterizations validate that InGeSiP3 undergoes a reversible Li-storage mechanism that involves intercalation, followed by conversion and alloy reactions, resulting in a reversible capacity of 1733 mA h g-1 with an initial Coulombic efficiency of 90%. Moreover, the InGeSiP3-based electrodes exhibit exceptional cycling stability, retaining an 1121 mA h g-1 capacity with a retention rate of ≈87% after 1500 cycles at 2000 mA g-1 and remarkable high-rate capability, achieving 882 mA h g-1 at 10 000 mA g-1. Inspired by the distinctive characteristic of high entropy, the synthesis is extended to high entropy GaCu (or Zn)InGeSiP5, CuZnInGeSiP5, GaCuZnInGeSiP6, InGeSiP2S (or Se), and InGeSiPSSe. This endeavor overcomes the immiscibility of different metals and non-metals, paving the way for the electrochemical energy storage application of high-entropy silicon-phosphides.

In Situ Transmission Electron Microscopy Methods for Lithium-Ion Batteries

In situ Transmission Electron Microscopy (TEM) stands as an invaluable instrument for the real-time examination of the structural changes in materials. It features ultrahigh spatial resolution and powerful analytical capability, making it significantly versatile across diverse fields. Particularly in the realm of Lithium-Ion Batteries (LIBs), in situ TEM is extensively utilized for real-time analysis of phase transitions, degradation mechanisms, and the lithiation process during charging and discharging. This review aims to provide an overview of the latest advancements in in situ TEM applications for LIBs. Additionally, it compares the suitability and effectiveness of two techniques: the open cell technique and the liquid cell technique. The technical aspects of both the open cell and liquid cell techniques are introduced, followed by a comparison of their applications in cathodes, anodes, solid electrolyte interphase (SEI) formation, and lithium dendrite growth in LIBs. Lastly, the review concludes by stimulating discussions on possible future research trajectories that hold potential to expedite the progression of battery technology.

Breaking Low-Strain and Deep-Potassiation Trade-Off in Alloy Anodes via Bonding Modulation for High-Performance K-Ion Batteries

Alloy anode materials have garnered unprecedented attention for potassium storage due to their high theoretical capacity. However, the substantial structural strain associated with deep potassiation results in serious electrode fragmentation and inadequate K-alloying reactions. Effectively reconciling the trade-off between low-strain and deep-potassiation in alloy anodes poses a considerable challenge due to the larger size of K-ions compared to Li/Na-ions. In this study, we propose a chemical bonding modulation strategy through single-atom modification to address the volume expansion of alloy anodes during potassiation. Using black phosphorus (BP) as a representative and generalizing to other alloy anodes, we established a robust P-S covalent bonding network via sulfur doping. This network exhibits sustained stability across discharge-charge cycles, elevating the modulus of K-P compounds by 74%, effectively withstanding the high strain induced by the potassiation process. Additionally, the bonding modulation reduces the formation energies of potassium phosphides, facilitating a deeper potassiation of the BP anode. As a result, the modified BP anode exhibits a high reversible capacity and extended operational lifespan, coupled with a high areal capacity. This work introduces a new perspective on overcoming the trade-off between low-strain and deep-potassiation in alloy anodes for the development of high-energy and stable potassium-ion batteries.

High-Rate SiO Lithium-Ion Battery Anode Enabled by Rationally Interfacial Hybrid Encapsulation Engineering

The development of a high-rate SiO lithium-ion battery anode is seriously limited by its low intrinsic conductivity, sluggish interfacial charge transfer (ICT), and unstable dynamic interface. To tackle the above issues, interfacial encapsulation engineering for effectively regulating the interfacial reaction and thus realizing a stable solid electrolyte interphase is significantly important. Hybrid coating, which aims to enhance the coupled e-/Li+ transport via the employment of dual layers, has emerged as a promising strategy. Herein, we construct a hybrid MXene-graphene oxide (GO) coating layer on the SiO microparticles. In the design, Ti3C2Tx MXene acts as a "bridge", which forms a close covalent connection with SiO and GO through Ti-O-Si and Ti-O-C bonds, respectively, thus greatly reducing the ICT resistance. Moreover, the Ti3C2Tx with rich surface groups (e.g., -OH, -F) and GO outer layers with an intertwined porous framework synergistically enable the pseudocapacitance dominated behavior, which is beneficial for fast lithium-ion storage. Accordingly, the as-made Si@MXene@GO anode exhibits considerably reinforced lithium-ion storage performance in terms of superior rate performance (1175.9 mA h g-1 at 5 A g-1) and long cycling stability (1087.6 mA h g-1 capacity retained after 1000 cycles at 2.0 A g-1). In-depth interfacial chemical composition analysis further reveals that an inorganically rich interphase with a gradient distribution of LiF and Li2O formed at the electrolyte/anode interface ensures mechanical stability during repeated cycles. This work paves a feasible way for maximizing the potential of SiO anodes toward fast-charging lithium-ion batteries.

Solution Combustion Synthesis of Submicron-Sized Titanium Niobium Oxide Anodes for High-Rate and Ultrastable Lithium-Ion Batteries

Recently, the development of high-rate performance lithium-ion batteries is crucial for the development of next-generation energy storage systems. Nanoarchitecturing of the electrode material is a common strategy to improve the effective Li+ diffusion transport rate. However, this method often results in a reduction of volumetric energy density and battery stability. In this work, we propose a different strategy by synthesizing submicron-sized Ti2Nb10O29 (s-TNO) as a durable high-rate anode material using a facile and scalable solution combustion method, eliminating the dependence nanoarchitectures. The s-TNO electrode material exhibits a large tunnel structure and an excellent pseudocapacitive performance. The results show that this electrode material delivers a commendable reversible capacity of 238.7 mAh g-1 at 0.5 C and retains 78.2% of its capacity after 10,000 cycles at 10 C. This work provides a valuable guide for the synthesis of submicron-structured electrode materials using the solution combustion method, particularly for high-capacity, high-rate, and high-stability electrode materials.

Textured Asymmetric Membrane Electrode Assemblies of Piezoelectric Phosphorene and Ti3C2Tx MXene Heterostructures for Enhanced Electrochemical Stability and Kinetics in LIBs

Black phosphorus with a superior theoretical capacity (2596 mAh g-1) and high conductivity is regarded as one of the powerful candidates for lithium-ion battery (LIB) anode materials, whereas the severe volume expansion and sluggish kinetics still impede its applications in LIBs. By contrast, the exfoliated two-dimensional phosphorene owns negligible volume variation, and its intrinsic piezoelectricity is considered to be beneficial to the Li-ion transfer kinetics, while its positive influence has not been discussed yet. Herein, a phosphorene/MXene heterostructure-textured nanopiezocomposite is proposed with even phosphorene distribution and enhanced piezo-electrochemical coupling as an applicable free-standing asymmetric membrane electrode beyond the skin effect for enhanced Li-ion storage. The experimental and simulation analysis reveals that the embedded phosphorene nanosheets not only provide abundant active sites for Li-ions, but also endow the nanocomposite with favorable piezoelectricity, thus promoting the Li-ion transfer kinetics by generating the piezoelectric field serving as an extra accelerator. By waltzing with the MXene framework, the optimized electrode exhibits enhanced kinetics and stability, achieving stable cycling performances for 1,000 cycles at 2 A g-1, and delivering a high reversible capacity of 524 mAh g-1 at - 20 ℃, indicating the positive influence of the structural merits of self-assembled nanopiezocomposites on promoting stability and kinetics.

Boosting the Cell Performance of the SiO/Cu and SiO/PPy Anodes via In-Situ Reduction/Oxidation Coating Strategies

Silicon monoxide (SiO) has attracted great attention due to its high theoretical specific capacity as an alternative material for conventional graphite anode, but its poor electrical conductivity and irreversible side reactions at the SiO/electrolyte interface seriously reduce its cycling stability. Here, to overcome the drawbacks, the dicharged SiO anode coated with Cu coating layer is elaborately designed by in-situ reduction method. Compared with the pristine SiO anode of lithium-ion battery (293 mAh g-1 at 0.5 A g-1 after 200 cycles), the obtained SiO/Cu composite presents superior cycling stability (1206 mAh g-1 at 0.5 A g-1 after 200 cycles). The tight combination of Cu particles and SiO significantly improves the conductivity of the composite, effectively inhibits the side-reaction between the active material and electrolyte. In addition, polypyrrole-coated SiO composites are further prepared by in-situ oxidation method, which delivers a high reversible specific capacity of 1311 mAh g-1 at 0.5 A g-1 after 200 cycles. The in-situ coating strategies in this work provide a new pathway for the development and practical application of high-performance silicon-based anode.

Interphase Regulation by Multifunctional Additive Empowering High Energy Lithium-Ion Batteries with Enhanced Cycle Life and Thermal Safety

High energy density lithium-ion batteries (LIBs) adopting high-nickel layered oxide cathodes and silicon-based composite anodes always suffer from unsatisfied cycle life and poor safety performance, especially at elevated temperatures. Electrode /electrolyte interphase regulation by functional additives is one of the most economic and efficacious strategies to overcome this shortcoming. Herein, cyano-groups (-CN) are introduced into lithium fluorinated phosphate to synthesize a novel multifunctional additive of lithium tetrafluoro (1,2-dihydroxyethane-1,1,2,2-tetracarbonitrile) phosphate (LiTFTCP), which endows high nickel LiNi0.8 Co0.1 Mn0.1 O2 /SiOx -graphite composite full cell with an ultrahigh cycle life and superior safety characteristics, by adding only 0.5 wt % LiTFTCP into a LiPF6 -carbonate baseline electrolyte. It is revealed that LiTFTCP additive effectively suppresses the HF generation and facilitates the formation of a robust and heat-resistant cyano-enriched CEI layer as well as a stable LiF-enriched SEI layer. The favorable SEI/CEI layers greatly lessen the electrode degradation, electrolyte consumption, thermal-induced gassing and total heat-releasing. This work illuminates the importance of additive molecular engineering and interphase regulation in simultaneously promoting the cycling and thermal safety of LIBs with high-nickel NCMxyz cathode and silicon-based composite anode.

Unraveling the Fundamental Mechanism of Interface Conductive Network Influence on the Fast-Charging Performance of SiO-Based Anode for Lithium-Ion Batteries

HIGHLIGHTS

Influence of interface conductive network on ionic transport and mechanical stability under fast charging is explored for the first time. The mitigation of interface polarization is precisely revealed by the combination of 2D modeling simulation and Cryo-TEM observation, which can be attributed to a higher fraction formation of conductive inorganic species in bilayer SEI, and primarily contributes to a linear decrease in ionic diffusion energy barrier. The improved stress dissipation presented by AFM and Raman shift is critical for the linear reduction in electrode residual stress and thickness swelling. Progress in the fast charging of high-capacity silicon monoxide (SiO)-based anode is currently hindered by insufficient conductivity and notable volume expansion. The construction of an interface conductive network effectively addresses the aforementioned problems; however, the impact of its quality on lithium-ion transfer and structure durability is yet to be explored. Herein, the influence of an interface conductive network on ionic transport and mechanical stability under fast charging is explored for the first time. 2D modeling simulation and Cryo-transmission electron microscopy precisely reveal the mitigation of interface polarization owing to a higher fraction of conductive inorganic species formation in bilayer solid electrolyte interphase is mainly responsible for a linear decrease in ionic diffusion energy barrier. Furthermore, atomic force microscopy and Raman shift exhibit substantial stress dissipation generated by a complete conductive network, which is critical to the linear reduction of electrode residual stress. This study provides insights into the rational design of optimized interface SiO-based anodes with reinforced fast-charging performance.

Crystal Engineering of Silica Anode Achieving Intrinsic Zero-Strain

Si-based anodes have large intrinsic volume expansion, which hinders their practicality and commercialization. To address this challenge, the design principle of intrinsic zero-strain anodes (① large intracrystalline cavities and ② strong bonds) is proposed, and silica with large intracrystalline cavities (SLIC) established by strong Si─O bonds ([SiO4 ] coordinate structures) is obtained and acts as an anode, achieving the intrinsic zero-strain feature first in silicon-based anodes. The phase structure of SLIC is maintained and the [SiO4 ] coordinate structure merely shows slight disorder during cycling. The feature stems from lithiation taking place by the solid-solution insertion reaction rather than the conventional conversion/alloying addition reactions, because the solid-solution insertion reaction for the SLIC has the lowest change in the Gibbs free energy. The SLIC anode demonstrates excellent cycling stability and high initial Coulombic efficiency (≈85%). Moreover, owing to the low working voltage (≈0.28 V) and relatively high specific capacity, the SLIC anode presents the highest gravimetric energy density among reported zero-/quasi-zero-strain anodes and high volumetric energy density (around twice as much as graphite). The universality of the designing principle is also validated. This work provides design guidelines for zero-strain anodes in next-generation batteries.

Defect Strategy in Solid-State Lithium Batteries

Solid-state lithium batteries (SSLBs) have great development prospects in high-security new energy fields, but face major challenges such as poor charge transfer kinetics, high interface impedance, and unsatisfactory cycle stability. Defect engineering is an effective method to regulate the composition and structure of electrodes and electrolytes, which plays a crucial role in dominating physical and electrochemical performance. It is necessary to summarize the recent advances regarding defect engineering in SSLBs and analyze the mechanism, thus inspiring future work. This review systematically summarizes the role of defects in providing storage sites/active sites, promoting ion diffusion and charge transport of electrodes, and improving structural stability and ionic conductivity of solid-state electrolytes. The defects greatly affect the electronic structure, chemical bond strength and charge transport process of the electrodes and solid-state electrolytes to determine their electrochemical performance and stability. Then, this review presents common defect fabrication methods and the specific role mechanism of defects in electrodes and solid-state electrolytes. At last, challenges and perspectives of defect strategies in high-performance SSLBs are proposed to guide future research.

Si-Based High-Entropy Anode for Lithium-Ion Batteries

Up to now, only a small portion of Si has been utilized in the anode for commercial lithium-ion batteries (LIBs) despite its high energy density. The main challenge of using micron-sized Si anode is the particle crack and pulverization due to the volume expansion during cycling. This work proposes a type of Si-based high-entropy alloy (HEA) materials with high structural stability for the LIB anode. Micron-sized HEA-Si anode can deliver a capacity of 971 mAhg-1 and retains 93.5% of its capacity after 100 cycles. In contrast, the silicon-germanium anode only retains 15% of its capacity after 20 cycles. This study has discovered that including HEA elements in Si-based anode can decrease its anisotropic stress and consequently enhance ductility at discharged state. By utilizing in situ X-ray diffraction and transmission electron microscopy analyses, a high-entropy transition metal doped Lix (Si/Ge) phase is found at lithiated anode, which returns to the pristine HEA phase after delithiation. The reversible lithiation and delithiation process between the HEA phases leads to intrinsic stability during cycling. These findings suggest that incorporating high-entropy modification is a promising approach in designing anode materials toward high-energy density LIBs.

Nanoporous silicon fiber networks in a composite anode for all-solid-state batteries with superior cycling performance

All-solid-state batteries comprising Si anodes are promising materials for energy storage in electronic vehicles because their energy density is approximately 1.7 times higher than that of graphite anodes. However, Si undergoes severe volume changes during cycling, resulting in the loss of electronic and ionic conduction pathways and rapid capacity fading. To address this challenge, we developed composite anodes with a nanoporous Si fiber network structure in sulfide-based solid electrolytes (SEs) and conductive additives. Nanoporous Si fibers were fabricated by electrospinning, followed by magnesiothermic reduction. The total pore volume of the fibers allowed pore shrinkage to compensate for the volumetric expansion of Li12Si7, thereby suppressing outward expansion and preserving the Si-SE (or conductive additive) interface. The network structure of the lithiated Si fibers compensates for electronic and ionic conduction pathways even to the partially delaminated areas, leading to increased Si utilization. The anodes exhibited superior performance, achieving an initial Coulombic efficiency of 71%, a reversible capacity of 1474 mAh g-1, and capacity retention of 85% after 40 cycles with an industrially acceptable areal capacity of 1.3 mAh cm-2. The proposed approach can reduce the constraint pressure during charging/discharging and may have practical applications in large-area all-solid-state batteries.

A B- and F-enriched buffering interphase enables a high-rate and high-stability SiOx/C anode

A facile, universal surface engineering strategy is proposed to address the volume expansion and slow kinetic issues encountered by SiOx/C anodes. A B-/F-enriched buffering interphase is introduced onto SiOx/C by thermal treatment of pre-adsorbed lithium salts at 400 °C. The as-prepared anode integrates both high-rate performance and long-term cycling durability.

Water-Soluble Polyamide Acid Binder with Fast Li+ Transfer Kinetics for Silicon Suboxide Anodes in Lithium-Ion Batteries

Silicon suboxide (SiOx) anodes have attracted considerable attention owing to their excellent cycling performance and rate capability compared to silicon (Si) anodes. However, SiOx anodes suffer from high volume expansion similar to Si anodes, which has been a challenge in developing suitable commercial binders. In this study, a water-soluble polyamide acid (WS-PAA) binder with ionic bonds was synthesized. The amide bonds inherent in the WS-PAA binder form a stable hydrogen bond with the SiOx anode and provide sufficient mechanical strength for the prepared electrodes. In addition, the ionic bonds introduced by triethylamine (TEA) induce water solubility and new Li+ transport channels to the binder, achieving enhanced electrochemical properties for the resulting SiOx electrodes, such as cycling and rate capability. The SiOx anode with the WS-PAA binder exhibited a high initial capacity of 1004.7 mAh·g-1 at a current density of 0.8 A·g-1 and a capacity retention of 84.9% after 200 cycles. Therefore, WS-PAA is a promising binder for SiOx anodes compared with CMC and SA.

Insights into the Coating Integrity and its Effect on the Electrochemical Performance of Core-Shell Structure SiOx @C Composite Anodes

Silicon-based anodes have been considered as ideal candidates for next-generation Li-ion batteries. However, the rapid cyclability decay due to significant volume expansion limits its commercialization. Besides, the instable interface further aggravates the degradation. Carbon coating is one effective way to improve the electrochemical performance.The coating integrity may be a critical index for core-shell structure electrode materials. Herein, the coating integrity of SiO_x @C composite is tested by a developed selective alkali dissolution, further quantitatively depicted by a proposed index of alkali solubility α. The effect of coating integrity on electrochemical performance reveals that SiO_x dissolution loss has a significant impact on the overall electrode structure stability and interface property. Because of the side reaction between uncoated active SiO_x and electrolyte, the quadratic decrease of initial coulombic efficiency and increase of solid electrolyte interphase thickness with the rise of alkali solubility are closely related to the generated F content induced by active material loss, further supported by the obvious linear rise of Li₂ SiF₆ fraction, leads to the linear increase of interface impedance and volume expansion rate, which may take primarily responsibility for the performance decay. This work propels the fundamental understanding on the interface failure mechnism and inspires rational high-performance electrode material design.

Insights into the Effect of SiO Particle Size on the Electrochemical Performance between Half and Full Cells for Li-Ion Batteries

Silicon monoxide (SiO) has attracted growing attention as one of the most promising anodes for high-energy-density lithium-ion batteries (LIBs), benefiting from relatively low volume expansion and superior cycling performance compared to bare silicon (Si). However, the size of the SiO particle for commercial application remains uncertain. Besides, the materials and concepts developed on the laboratory level in half cells are quite different from what is necessary for practical operation in full cells. Herein, we investigate the electrochemical performance of SiO with different particle sizes between half cells and full cells. The SiO with larger particle size exhibits worse electrochemical performance in the half cell, whereas it demonstrates excellent cycling stability with a high capacity retention of 91.3% after 400 cycles in the full cell. The reasons for the differences in their electrochemical performance between half cells and full cells are further explored in detail. The SiO with larger particle size possessing superior electrochemical performance in full cells benefits from consuming less electrolyte and not being easier to aggregate. It indicates that the SiO with larger particle size is recommended for commercial application and part of the information provided from half cells may not be advocated to predict the cycling performances of the anode materials. The analysis based on the electrochemical performance of the SiO between half cells and full cells gives fundamental insight into further Si-based anode research.

Well-Dispersed Fe Nanoclusters for Effectively Increasing the Initial Coulombic Efficiency of the SiO Anode

An efficient surface modification strategy is proposed to significantly increase the initial Coulombic efficiency (ICE) of SiO anode material. The SiO@Fe material with the Fe nanocluster homogeneously decorating on the SiO surface is successfully prepared by a chemical vapor deposition process. The well-dispersed Fe nanoclusters realize an Ohmic contact with lithium silicates, the commonly regarded irreversible lithiation product, which effectively lowers the electron conduction barriers and promotes the concomitant lithium-ion release of the lithium silicates upon the delithiation process, increasing the ICE of the SiO anode. The prepared SiO@Fe exhibits a much higher ICE of 87.2% compared to 64.4% of pristine SiO, with the largest increment (23%) never reported, except for the prelithiation, and delivers significantly enhanced cycling and rate performance. These findings provide an effective way to convert the "inert" phase to "active" which essentially increases the ICE of the electrode.

An Elastic Cross-Linked Binder for Silicon Anodes in Lithium-Ion Batteries with a High Mass Loading

Due to the urgent demand for lithium-ion batteries (LIBs) with a high energy density, silicon (Si) possessing an ultrahigh capacity has aroused wide attention. However, its practical application is seriously hindered by enormous volume changes of the Si anode during cycling. Developing novel binders suitable for the Si anode has proven to be an effective strategy to improve its electrochemical performance. Herein, we constructed a three-dimensional network binder, in which the polyacrylic acid (PAA) long chains are cross-linked with one kind of amino acid, lysine (Lys). The abundant polar groups in PAA/Lys enable it to tightly adhere to the Si particles via hydrogen bonds, and the cross-linked structure prevents irreversible slipping of the PAA chains upon volume variation of the particles. The Si used was obtained from a sustainable route by recycling photovoltaic waste silicon. With high elasticity and strong adhesion, the PAA/Lys binder can effectively keep the structural integrity of the Si electrode and improve its electrochemical performance. The Si electrode using the PAA/Lys binder exhibits a good cycling stability (1008 mAh g^-1 at 2 A g^-1 after 250 cycles). Even with a high mass loading of 3.03 mg cm^-2, the Si anode can remain stable for 100 cycles at a high fixed areal capacity of 3.03 mAh cm^-2. This work gives a practical method to make stable Si electrodes using sustainable Si source and environmentally friendly amino acid-based binders.

Templated Synthesis of SiO2 Nanotubes for Lithium-Ion Battery Applications: An In Situ (Scanning) Transmission Electron Microscopy Study

One of the weaknesses of silicon-based batteries is the rapid deterioration of the charge-storage capacity with increasing cycle numbers. Pure silicon anodes tend to suffer from poor cycling ability due to the pulverization of the crystal structure after repeated charge and discharge cycles. In this work, we present the synthesis of a hollow nanostructured SiO₂ material for lithium-ion anode applications to counter this drawback. To improve the understanding of the synthesis route, the crucial synthesis step of removing the ZnO template core is shown using an in situ closed gas-cell sample holder for transmission electron microscopy. A direct visual observation of the removal of the ZnO template from the SiO₂ shell is yet to be reported in the literature and is a critical step in understanding the mechanism by which these hollow nanostructures form from their core-shell precursors for future electrode material design. Using this unique technique, observation of dynamic phenomena at the individual particle scale is possible with simultaneous heating in a reactive gas environment. The electrochemical benefits of the hollow morphology are demonstrated with exceptional cycling performance, with capacity increasing with subsequent charge-discharge cycles. This demonstrates the criticality of nanostructured battery materials for the development of next-generation Li⁺-ion batteries.

Optimal Microstructure of Silicon Monoxide as the Anode for Lithium-Ion Batteries

Because of its metastable nature, silicon monoxide (SiO) consists of Si nanodomains in an amorphous matrix of SiO₂. The microstructure of SiO, including SiO₂, Si domains, and interphase (SiO_x) between domains, was modified via an annealing treatment in argon gas and thoroughly characterized by in-situ and ex-situ X-ray diffraction, pair distribution function, and electron energy loss spectroscopy. Two microstructure transformation routes were observed during the annealing process: (1) at a temperature of <800 °C, the annealing treatment was found to affect mainly the structural conformation of the amorphous SiO₂ matrix and the interphase, while (2) an annealing temperature of >800 °C led to significant Si nanodomain growth. We found that the microstructure has a great impact on the electrochemical performance of SiO. The optimized microstructure of SiO appears to be achieved through annealing treatment at 800 °C or less, which results in interphase (SiO_x) reduction without causing significant Si domain growth. This work provides a deep insight into the domain and interphase transformation of SiO upon heat treatment. The improved understanding of the relationship between SiO microstructure and its electrochemical behavior will enable proper design and development of high-energy SiO for lithium-ion batteries.

Engineering High-Performance SiOx Anode Materials with a Titanium Oxynitride Coating for Lithium-Ion Batteries

Micron-sized silicon oxide (SiOx) has been regarded as a promising anode material for new-generation lithium-ion batteries due to its high capacity and low cost. However, the distinct volume expansion during the repeated (de)lithiation process and poor conductivity can lead to structural collapse of the electrode and capacity fading. In this study, SiOx anode materials coated with TiO0.6N0.4 layers are fabricated by a facile solvothermal and thermal reduction technique. The TiO0.6N0.4 layers are homogeneously dispersed on SiOx particles and form an intimate contact. The TiO0.6N0.4 layers can enhance the conductivity and suppress volume expansion of the SiOx anode, which facilitate ion/electron transport and maintain the integrity of the overall electrode structure. The as-prepared SiOx-TiON-200 composites demonstrate a high reversible capacity of 854 mAh g-1 at 0.5 A g-1 with a mass loading of 2.0 mg cm-2 after 250 cycles. This surface modification technique could be extended to other anodes with low conductivity and large volume expansion for lithium-ion batteries.

Endothermic Dehydrogenation-Driven Preventive Magnesiation of SiO for High-Performance Lithium Storage Materials

Silicon monoxide (SiO)-based materials have gained much attention as high-capacity lithium storage materials based on their high capacity and stable capacity retention. However, low initial Coulombic efficiency associated with the irreversible electrochemical reaction of the amorphous SiO₂ phase in SiO inhibits the wide usage of SiO-based anode materials for lithium-ion batteries. Magnesiation of SiO is one of the most promising solutions to improve the initial efficiency of SiO-based anode materials. Herein, we demonstrate that endothermic dehydrogenation-driven magnesiation of SiO employing MgH₂ enhanced the initial Coulombic efficiency of 89.5% with much improved long-term cycle performance over 300 cycles compared to the homologue prepared by magnesiation of SiO with Mg and pristine SiO. High-resolution transmission electron microscopy with thermogravimetry-differential scanning calorimetry revealed that the endothermic dehydrogenation of MgH₂ suppressed the sudden temperature rise during magnesiation of SiO, thereby inhibiting the coarsening of the active Si phase in the resulting Si/Mg₂SiO₄ nanocomposite.

Assembly: A Key Enabler for the Construction of Superior Silicon-Based Anodes

Silicon (Si) is regarded as the most promising anode material for high-energy lithium-ion batteries (LIBs) due to its high theoretical capacity, and low working potential. However, the large volume variation during the continuous lithiation/delithiation processes easily leads to structural damage and serious side reactions. To overcome the resultant rapid specific capacity decay, the nanocrystallization and compound strategies are proposed to construct hierarchically assembled structures with different morphologies and functions, which develop novel energy storage devices at nano/micro scale. The introduction of assembly strategies in the preparation process of silicon-based materials can integrate the advantages of both nanoscale and microstructures, which significantly enhance the comprehensive performance of the prepared silicon-based assemblies. Unfortunately, the summary and understanding of assembly are still lacking. In this review, the understanding of assembly is deepened in terms of driving forces, methods, influencing factors and advantages. The recent research progress of silicon-based assembled anodes and the mechanism of the functional advantages for assembled structures are reviewed from the aspects of spatial confinement, layered construction, fasciculate structure assembly, superparticles, and interconnected assembly strategies. Various feasible strategies for structural assembly and performance improvement are pointed out. Finally, the challenges and integrated improvement strategies for assembled silicon-based anodes are summarized.

Facile Synthesis of Hybrid Anodes with Enhanced Lithium-Storage Performance Realized by a "Synergistic Effect"

Alloying-type anodes including Si- and Sn-based materials are considered the most exploitable anodes for high-performance lithium-ion batteries. However, problems of poor kinetics properties and structural failures such as grain pulverization and coarsening hinder their large-scale application. Herein, SnO₂/Si@graphite hybrid anodes, with nano-SnO₂ and nano-Si thoroughly mixed with each other and loaded onto graphite flakes, have been prepared by a facile ball milling method. Attributed to the "synergistic effect" between SnO₂ and Si, the mechanical stability and kinetics properties can be remarkably enhanced. Furthermore, graphite substrate supplies a fast electrically conductive path and buffers the volume expansion of active particles. Accordingly, SnO₂/Si@graphite delivers 798.9 mAh g^-1 at 200 mA g^-1 and maintains 550.8 mAh g^-1 after 1000 cycles at 1 A g^-1 in half cells. Impressively, a high energy density of 431.4 Wh kg^-1 (based on the mass of anode and cathode) can be obtained in full cells when paired with the NCM622 cathode. This work presents an effective strategy to exploit high-performance alloying-type anodes for LIBs by designing hybrid materials with multiple active components.

A highly stable pre-lithiated SiOx anode coated with a "salt-in-polymer" layer

An artificial "salt-in-polymer" SEI, composed of poly-(1,3-dioxolane) and high-modulus fluorinated products generated from the in situ decomposition of Li salts, was constructed on the surface of Li-MSiO_x particles. This LiF-rich SEI helps to maintain the structural integrity of Li-MSiO_x particles and improves the Li storage reversibility of the Li-MSiO_x anode.

Lithium/Boron Co-doped Micrometer SiOx as Promising Anode Materials for High-Energy-Density Li-Ion Batteries

The carbon-coated silicon monoxide (SiO_x@C) has been considered as one of the most promising high-capacity anodes for the next-generation high-energy-density lithium-ion batteries (LIBs). However, the relatively low initial Coulombic efficiency (ICE) and the still existing huge volume expansion during repeated lithiation/delithiation cycling remain the greatest challenges to its practical application. Here, we developed a lithium and boron (Li/B) co-doping strategy to efficiently enhance the ICE and alleviate the volume expansion or pulverization of SiO_x@C anodes. The in situ generated Li silicates (Li_xSiO_y) by Li doping will reduce the active Li loss during the initial cycling and enhance the ICE of SiO_x@C anodes. Meanwhile, B doping works to promote the Li⁺ diffusion and strengthen the internal bonding networks within SiO_x@C, enhancing its resistance to cracking and pulverization during cycling. As a result, the enhanced ICE (83.28%), suppressed volume expansion, and greatly improved cycling (85.4% capacity retention after 200 cycles) and rate performance could be achieved for the Li/B co-doped SiO_x@C (Li/B-SiO_x@C) anodes. Especially, the Li/B-SiO_x@C and graphite composite anodes with a capacity of 531.5 mA h g^-1 were demonstrated to show an ICE of 90.1% and superior cycling stability (90.1% capacity retention after 250 cycles), which is significant for the practical application of high-energy-density LIBs.

Advances of Synthesis Methods for Porous Silicon-Based Anode Materials

Silicon (Si)-based anode materials have been the promising candidates to replace commercial graphite, however, there are challenges in the practical applications of Si-based anode materials, including large volume expansion during Li⁺ insertion/deinsertion and low intrinsic conductivity. To address these problems existed for applications, nanostructured silicon materials, especially Si-based materials with three-dimensional (3D) porous structures have received extensive attention due to their unique advantages in accommodating volume expansion, transportation of lithium-ions, and convenient processing. In this review, we mainly summarize different synthesis methods of porous Si-based materials, including template-etching methods and self-assembly methods. Analysis of the strengths and shortages of the different methods is also provided. The morphology evolution and electrochemical effects of the porous structures on Si-based anodes of different methods are highlighted.