Evidence of covalent synergy in silicon-sulfur-graphene yielding highly efficient and long-life lithium-ion batteries

Both Resilience and Adhesivity Define Solid Electrolyte Interphases for a High Performance Anode

Solid electrolyte interphases (SEIs) are sought to protect high-capacity anodes, which suffer from severe volume changes and fast degradations. The previously proposed effective SEIs were of high strength yet abhesive, inducing a yolk-shell structure to decouple the rigid SEI from the anode for accommodating the volume change. Ambivalently, the interfacial void-evolved electro-chemo-mechanical vulnerabilities become inherent defects. Here, we establish a new rationale for SEIs that resilience and adhesivity are both requirements and pioneer a design of a resilient yet adhesive SEI (re-ad-SEI), integrated into a conjugated surface bilayer structure. The re-ad-SEI and its protected particles exhibit excellent stability almost free from the thickening of SEI and the particle pulverization during cycling. More promisingly, the dynamically bonded intact SEI-anode interfaces enable a high-efficiency ion transport and provide a unique mechanical confinement effect for structural integrity of anodes. The high Coulombic efficiency (>99.8%), excellent cycling stability (500 cycles), and superior rate performance have been demonstrated in microsized Si-based anodes.

Strong p-d Orbital Hybridization on Bismuth Nanosheets for High Performing CO2 Electroreduction

Single-atom alloys (SAAs) show great potential for a variety of electrocatalytic reactions. However, the atomic orbital hybridization effect of SAAs on the electrochemical reactions is unclear yet. Herein, the in situ confinement of vanadium/molybdenum/tungsten atoms on bismuth nanosheet is shown to create SAAs with rich grain boundaries, respectively. With the detailed analysis of microstructure and composition, the strong p-d orbital hybridization between bismuth and vanadium enables the exceptional electrocatalytic performance for carbon dioxide (CO2 ) reduction with the Faradaic efficiency nearly 100% for C1 products in a wide potential range from -0.6 to -1.4 V, and a long-term electrolysis stability for 90 h. In-depth in situ investigations with theoretical computations reveal that the electron delocalization toward vanadium atoms via the p-d orbital hybridization evokes the bismuth active centers for efficient CO2 activation via the σ-donation of O-to-Bi, thus reduces protonation energy barriers for formate production. With such fundamental understanding, SAA electrocatalyst is employed to fabricated the solar-driven electrolytic cell of CO2 reduction and 5-hydroxymethylfurfural oxidation, achieving an outstanding 2,5-furandicarboxylic acid yield of 90.5%. This study demonstrates a feasible strategy to rationally design advanced SAA electrocatalysts via the basic principles of p-d orbital hybridization.

2D Layered Materials for Fast-Charging Lithium-Ion Battery Anodes

The development of electric vehicles has received worldwide attention in the background of reducing carbon emissions, wherein lithium-ion batteries (LIBs) become the primary energy supply systems. However, commercial graphite-based anodes in LIBs currently confront significant difficulty in enduring ultrahigh power input due to the slow Li+ transport rate and the low intercalation potential. This will, in turn, cause dramatic capacity decay and lithium plating. The 2D layered materials (2DLMs) recently emerge as new fast-charging anodes and hold huge promise for resolving the problems owing to the synergistic effect of a lower Li+ diffusion barrier, a proper Li+ intercalation potential, and a higher theoretical specific capacity with using them. In this review, the background and fundamentals of fast-charging for LIBs are first introduced. Then the research progress recently made for 2DLMs used for fast-charging anodes are elaborated and discussed. Some emerging research directions in this field with a short outlook on future studies are further discussed.

Atomic bismuth induced ensemble sites with indium towards highly efficient and stable electrocatalytic reduction of carbon dioxide

Structural reconstruction is commonly observed during electrocatalytic CO₂ reduction (CO₂RR) process. However, the proper modulation of interface and defect sites remains challenging with the mechanism understanding to realize the favorable electrocatalysis. Herein, the atomic bridging of bismuth with indium atoms is elaborately designed for improving electrocatalysis of CO₂RR via electrochemical reduction and in situ anchoring strategy. As revealed by in situ structure analysis and theoretical studies, the ensemble sites supported on carbon matrix enable the charge density gradient to significantly promote the adsorption of *OCHO intermediate by the regulation of σ bonding and π* back-donation. Consequently, such unique electrocatalyst achieves the high formate faradaic efficiency of 95.1% over the entire potential range tested and the long-lived stability for 9 d. With coupling of CO₂RR, the solar-driven full cell demonstrates the spontaneous production of formate and 2,5-furandicarboxylic acid via the efficient oxidation of 5-hydroxymethylfurfural with an outstanding yield of 88.2%, highlighting the impressive solar-to-fuel conversion selectivity. Monitoring and understanding the intrinsic active sites of biatomic bridge are crucial to elucidate the synergic electrocatalysis for rationally designing high-performance electrocatalysts.

Rechargeable Batteries for Grid Scale Energy Storage

Ever-increasing global energy consumption has driven the development of renewable energy technologies to reduce greenhouse gas emissions and air pollution. Battery energy storage systems (BESS) with high electrochemical performance are critical for enabling renewable yet intermittent sources of energy such as solar and wind. In recent years, numerous new battery technologies have been achieved and showed great potential for grid scale energy storage (GSES) applications. However, their practical applications have been greatly impeded due to the gap between the breakthroughs achieved in research laboratories and the industrial applications. In addition, various complex applications call for different battery performances. Matching of diverse batteries to various applications is required to promote practical energy storage research achievement. This review provides in-depth discussion and comprehensive consideration in the battery research field for GSES. The overall requirements of battery technologies for practical applications with key parameters are systematically analyzed by generating standards and measures for GSES. We also discuss recent progress and existing challenges for some representative battery technologies with great promise for GSES, including metal-ion batteries, lead-acid batteries, molten-salt batteries, alkaline batteries, redox-flow batteries, metal-air batteries, and hydrogen-gas batteries. Moreover, we emphasize the importance of bringing emerging battery technologies from academia to industry. Our perspectives on the future development of batteries for GSES applications are provided.

Easy Diameter Tuning of Silicon Nanowires with Low-Cost SnO2-Catalyzed Growth for Lithium-Ion Batteries

Silicon nanowires are appealing structures to enhance the capacity of anodes in lithium-ion batteries. However, to attain industrial relevance, their synthesis requires a reduced cost. An important part of the cost is devoted to the silicon growth catalyst, usually gold. Here, we replace gold with tin, introduced as low-cost tin oxide nanoparticles, to produce a graphite-silicon nanowire composite as a long-standing anode active material. It is equally important to control the silicon size, as this determines the rate of decay of the anode performance. In this work, we demonstrate how to control the silicon nanowire diameter from 10 to 40 nm by optimizing growth parameters such as the tin loading and the atmosphere in the growth reactor. The best composites, with a rich content of Si close to 30% wt., show a remarkably high initial Coulombic efficiency of 82% for SiNWs 37 nm in diameter.

Strategy for enhanced performance of silicon nanoparticle anodes for lithium-ion batteries

The modification of silicon nanoparticles for lithium-ion battery anode materials has been a hot exploration subject in light of their excellent volume buffering performance. However, huge volume expansion leads to an unstable solid electrolyte interface (SEI) layer on the surface of the silicon anode material, resulting in short cell cycle life, which is an important factor limiting the application of silicon nanoparticles. Herein, a dual protection strategy to improve the cycling stability of commercial silicon nanoparticles is demonstrated. Specifically, the Si/s-C@TiO2 composite was produced by the hydrothermal method to achieve the embedding of commercial silicon nanoparticles in spherical carbon and the coating of the amorphous TiO2 shell on the outer surface. Buffering of silicon nanoparticle volume expansion by spherical carbon and also the stabilization of the TiO2 shell with high mechanical strength on the surface constructed a stable outer surface SEI layer of the new Si/s-C@TiO2 electrode during longer cycling. In addition, the spherical carbon and lithiated TiO2 further enhanced the electronic and ionic conductivity of the composite. Electrochemical measurements showed that the Si/s-C@TiO2 composite exhibited excellent lithium storage performance (780 mA h g-1 after 100 cycles at a current density of 0.2 A g-1 with a coulombic efficiency of 99%). Our strategy offers new ideas for the production of high stability and high-performance anode materials for lithium-ion batteries.

Nano-crumples induced Sn-Bi bimetallic interface pattern with moderate electron bank for highly efficient CO2 electroreduction

CO₂ electroreduction reaction offers an attractive approach to global carbon neutrality. Industrial CO₂ electrolysis towards formate requires stepped-up current densities, which is limited by the difficulty of precisely reconciling the competing intermediates (COOH* and HCOO*). Herein, nano-crumples induced Sn-Bi bimetallic interface-rich materials are in situ designed by tailored electrodeposition under CO₂ electrolysis conditions, significantly expediting formate production. Compared with Sn-Bi bulk alloy and pure Sn, this Sn-Bi interface pattern delivers optimum upshift of Sn p-band center, accordingly the moderate valence electron depletion, which leads to weakened Sn-C hybridization of competing COOH* and suitable Sn-O hybridization of HCOO*. Superior partial current density up to 140 mA/cm² for formate is achieved. High Faradaic efficiency (>90%) is maintained at a wide potential window with a durability of 160 h. In this work, we elevate the interface design of highly active and stable materials for efficient CO₂ electroreduction.

Reinforced concrete inspired Si/rGO/cPAN hybrid electrode: highly improved lithium storage via Si electrode nanoarchitecture engineering

Electrode nanoarchitecture engineering is a transformative way to improve the structural stability and build robust transport charge pathways for high-capacity silicon in lithium ion batteries (LIBs). However, the violent expansion of silicon during the lithiation/delithiation process is the chief reason for its limited industrialization. Here, we fabricated an integrated electrode structure using polyacrylonitrile (PAN) and graphene oxide (GO) inspired by reinforced concrete. Based on low-temperature annealing, cyclized PAN was assembled on the surface of silicon nanoparticles and tightly combined with reduced graphene oxide (rGO), which could construct stable and efficient transport channels for electrons and lithium ions and address the issues of electrode structure and interface stability. The resultant Si/rGO/cPAN (RC-Si) as the LIB anode exhibits exceptional combined performances including extraordinary mechanical properties, excellent cycling stability (∼1150 mA h g^-1 at 2 A g^-1 over 500 cycles), superior rate capability (∼600 mA h g^-1 at 12 A g^-1), and high areal capacity (∼5.6 mA h cm^-2 at 0.5 mA cm^-2). The novel electrode design concept is promising to promote the practical application of silicon anodes and open a new avenue to develop other high-capacity anodes for high-performance batteries.

Sea Urchin-like Si@MnO2@rGO as Anodes for High-Performance Lithium-Ion Batteries

Si is a promising material for applications as a high-capacity anode material of lithium-ion batteries. However, volume expansion, poor electrical conductivity, and a short cycle life during the charging/discharging process limit the commercial use. In this paper, new ternary composites of sea urchin-like Si@MnO₂@reduced graphene oxide (rGO) prepared by a simple, low-cost chemical method are presented. These can effectively reduce the volume change of Si, extend the cycle life, and increase the lithium-ion battery capacity due to the dual protection of MnO₂ and rGO. The sea urchin-like Si@MnO₂@rGO anode shows a discharge specific capacity of 1282.72 mAh g⁻¹ under a test current of 1 A g⁻¹ after 1000 cycles and excellent chemical performance at different current densities. Moreover, the volume expansion of sea urchin-like Si@MnO₂@rGO anode material is ~50% after 150 cycles, which is much less than the volume expansion of Si (300%). This anode material is economical and environmentally friendly and this work made efforts to develop efficient methods to store clean energy and achieve carbon neutrality.

Microscale Silicon-Based Anodes: Fundamental Understanding and Industrial Prospects for Practical High-Energy Lithium-Ion Batteries

To accelerate the commercial implementation of high-energy batteries, recent research thrusts have turned to the practicality of Si-based electrodes. Although numerous nanostructured Si-based materials with exceptional performance have been reported in the past 20 years, the practical development of high-energy Si-based batteries has been beset by the bias between industrial application with gravimetrical energy shortages and scientific research with volumetric limits. In this context, the microscale design of Si-based anodes with densified microstructure has been deemed as an impactful solution to tackle these critical issues. However, their large-scale application is plagued by inadequate cycling stability. In this review, we present the challenges in Si-based materials design and draw a realistic picture regarding practical electrode engineering. Critical appraisals of recent advances in microscale design of stable Si-based materials are presented, including interfacial tailoring of Si microscale electrode, surface modification of SiO_x microscale electrode, and structural engineering of hierarchical microscale electrode. Thereafter, other practical metrics beyond active material are also explored, such as robust binder design, electrolyte exploration, prelithiation technology, and thick-electrode engineering. Finally, we provide a roadmap starting with material design and ending with the remaining challenges and integrated improvement strategies toward Si-based full cells.

1000 Wh L-1 lithium-ion batteries enabled by crosslink-shrunk tough carbon encapsulated silicon microparticle anodes

Microparticulate silicon (Si), normally shelled with carbons, features higher tap density and less interfacial side reactions compared to its nanosized counterpart, showing great potential to be applied as high-energy lithium-ion battery anodes. However, localized high stress generated during fabrication and particularly, under operating, could induce cracking of carbon shells and release pulverized nanoparticles, significantly deteriorating its electrochemical performance. Here we design a strong yet ductile carbon cage from an easily processing capillary shrinkage of graphene hydrogel followed by precise tailoring of inner voids. Such a structure, analog to the stable structure of plant cells, presents 'imperfection-tolerance' to volume variation of irregular Si microparticles, maintaining the electrode integrity over 1000 cycles with Coulombic efficiency over 99.5%. This design enables the use of a dense and thick (3 mAh cm-2) microparticulate Si anode with an ultra-high volumetric energy density of 1048 Wh L-1 achieved at pouch full-cell level coupled with a LiNi0.8Co0.1Mn0.1O2 cathode.

Double-Shelled Hollow SiO2 @N-C Nanofiber Boosts the Lithium Storage Performance of [PMo12 O40 ]3

Polyoxometalates (POMs)-based materials, with high theoretical capacities and abundant reversible multi-electron redox properties, are considered as promising candidates in lithium-ion storage. However, the poor electronic conductivity, low specific surface area and high solubility in the electrolyte limited their practical applications. Herein, a double-shelled hollow PMo₁₂ -SiO₂ @N-C nanofiber (PMo₁₂ -SiO₂ @N-C, where PMo₁₂ is [PMo₁₂ O₄₀ ]^3- , N-C is nitrogen-doped carbon) was fabricated for the first time by combining coaxial electrospinning technique, thermal treatment and electrostatic adsorption. As an anode material for LIBs, the PMo₁₂ -SiO₂ @N-C delivered an excellent specific capacity of 1641 mA h g^-1 after 1000 cycles under 2 A g^-1 . The excellent electrochemical performance benefited from the unique double-shelled hollow structure of the material, in which the outermost N-C shell cannot only hinder the agglomeration of PMo₁₂ , but also improve its electronic conductivity. The SiO₂ inner shell can efficiently avoid the loss of active components. The hollow structure can buffer the volume expansion and accelerate Li⁺ diffusion during lithiation/delithiation process. Moreover, PMo₁₂ can greatly reduce charge-resistance and facilitate electron transfer of the entire composites, as evidenced by the EIS kinetics study and lithium-ion diffusion analysis. This work paves the way for the fabrication of novel POM-based LIBs anode materials with excellent lithium storage performance.

Covalent Coupling-Stabilized Transition-Metal Sulfide/Carbon Nanotube Composites for Lithium/Sodium-Ion Batteries

Transition-metal sulfides (TMSs) powered by conversion and/or alloying reactions are considered to be promising anode materials for advanced lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs). However, the limited electronic conductivity and large volume expansion severely hinder their practical application. Herein, we report a covalent coupling strategy for TMS-based anode materials using amide linkages to bind TMSs and carbon nanotubes (CNTs). In the synthesis, the thiourea acts as not only the capping agent for morphology control but also the linking agent for the covalent coupling. As a proof of concept, the covalently coupled ZnS/CNT composite (CC-ZnS/CNT) has been prepared, with ZnS nanoparticles (∼10 nm) tightly anchored on CNT bundles. The compact ZnS-CNT heterojunctions are greatly beneficial to facilitating the electron/ion transfer and ensuring structural stability. Due to the strong coupling interaction between ZnS and CNTs, the composite presents prominent pseudocapacitive behavior and highly reversible electrochemical processes, thus leading to superior long-term stability and excellent rate capability, delivering reversible capacities of 333 mAh g^-1 at 2 A g^-1 over 4000 cycles for LIBs and 314 mAh g^-1 at 5 A g^-1 after 500 cycles for SIBs. Consequently, CC-ZnS/CNT exhibits great competence for applications in LIBs and SIBs, and the covalent coupling strategy is proposed as a promising approach for designing high-performance anode materials.

When Silicon Materials Meet Natural Sources: Opportunities and Challenges for Low-Cost Lithium Storage

The manipulation of progressive lithium-ion batteries (LIBs) with high energy density, low cost, and long-term cycling stability is of high priority to meet the growing demands for next-generation energy storage devices. Silicon (Si) has been receiving marvelous attention as a promising anode material for rechargeable LIBs, due to its high theoretical gravimetric capacity and low cost. Si is the second most abundant element in the earth crust in the form of silicates, so it is the most cost-effective element as an anode material in next-generation LIBs. In this review, different natural sources such as rice husk, sugar cane bagasse, bamboo, reed plant, sand, halloysite, and different waste sources such as waste of the solar power industry, fly ash, straw ash, and other industrial waste that can give rise to different nanostructured Si are systematically summarized. In addition, different synthesis methods of fabricating nanostructured Si are reviewed as well as including magnesiothermic reduction, etching methods, ball milling, and chemical vapor deposition. The advantages and disadvantages of these kind of synthesis methods are discussed as well. Furthermore, the opportunities and challenges of nano-Si are also discussed.

Ultrasonic Generation of Thiyl Radicals: A General Method of Rapidly Connecting Molecules to a Range of Electrodes for Electrochemical and Molecular Electronics Applications

Herein, we report ultrasonic generation of thiyl radicals as a general method for functionalizing a range of surfaces with organic molecules. The method is simple, rapid, can be utilized at ambient conditions and involves sonicating a solution of disulfide molecules, homolytically cleaving S-S bonds and generating thiyl radicals that react with the surfaces by forming covalently bound monolayers. Full molecular coverages on conducting oxides (ITO), semiconductors (Si-H), and carbon (GC) electrode surfaces can be achieved within a time scale of 15-90 min. The suitability of this method to connect the same molecule to different electrodes enabled comparing the conductivity of single molecules and the electrochemical electron transfer kinetics of redox active monolayers as a function of the molecule-electrode contact. We demonstrate, using STM break-junction technique, single-molecule heterojunction comprising Au-molecule-ITO and Au-molecule-carbon circuits. We found that despite using the same molecule, the single-molecule conductivity of Au-molecule-carbon circuits is about an order of magnitude higher than that of Au-molecule-ITO circuits. The same trend was observed for electron transfer kinetics, measured using electrochemical impedance spectroscopy for ferrocene-terminated monolayers on carbon and ITO. This suggests that the interfacial bond between different electrodes and the same molecule can be used to tune the conductivity of single-molecule devices and to control the rate of charge transport in redox active monolayers, opening prospects for relating various types of interfacial charge-transfer rate processes.

Confinement of PMo12 in hollow SiO2-PMo12@rGO nanospheres for high-performance lithium storage

High-performance lithium storage was achieved by the confinement of PMo12 in hollow SiO2-PMo12@rGO nanocomposites.

Two-Dimensional Silicon/Carbon from Commercial Alloy and CO2 for Lithium Storage and Flexible Ti3C2Tx MXene-Based Lithium-Metal Batteries

Silicon has been considered as the most promising anode candidate for next-generation lithium-ion batteries. However, the fast capacity decay caused by huge volume expansion and low electronic conductivity limit the electrochemical performance. Herein, atomic distributed, air-stable, layer-by-layer-assembled Si/C (L-Si/C) is designed and in situ constructed from commercial micron-sized layered CaSi₂ alloy with the greenhouse gas CO₂. The inner structure of Si as well as the content and graphitization of C can be regulated by simply adjusting the reaction conditions. The rationally designed layered structure can enhance electronic conductivity and mitigate volume change without disrupting the carbon layer or destroying the solid electrolyte interface. Moreover, the single-layer Si and C can enhance lithium-ion transport in active materials. With these advantages, L-Si/C anode delivers an 82.85% capacity retention even after 3200 cycles and superior rate performance. The battery-capacitance dual-model mechanism is certified via quantitative kinetics measurement. Besides, the self-standing architecture is designed via assembling L-Si/C and MXene. Lithiophilic L-Si/C can guide homogeneous Li deposition with alleviated volume change. With the MXene/L-Si/C host for lithium-metal batteries, an ultralong life span up to 500 h in a carbonate-based electrolyte is achieved. A full cell with a high-energy 5 V LiNi_0.5Mn_1.5O₄ cathode is constructed to verify the practicality of L-Si/C and MXene/L-Si/C. The rational design of a special layer structure may propose a strategy for other materials and energy storage systems.

Covalent Linkages of Molecules and Proteins to Si-H Surfaces Formed by Disulfide Reduction

Thiols and disulfide contacts have been, for decades, key for connecting organic molecules to surfaces and nanoclusters as they form self-assembled monolayers (SAMs) on metals such as gold (Au) under mild conditions. In contrast, they have not been similarly deployed on Si owing to the harsh conditions required for monolayer formation. Here, we show that SAMs can be simply formed by dipping Si-H surfaces into dilute solutions of organic molecules or proteins comprising disulfide bonds. We demonstrate that S-S bonds can be spontaneously reduced on Si-H, forming covalent Si-S bonds in the presence of traces of water, and that this grafting can be catalyzed by electrochemical potential. Cyclic disulfide can be spontaneously reduced to form complete monolayers in 1 h, and the reduction can be catalyzed electrochemically to form full surface coverages within 15 min. In contrast, the kinetics of SAM formation of the cyclic disulfide molecule on Au was found to be three-fold slower than that on Si. It is also demonstrated that dilute thiol solutions can form monolayers on Si-H following oxidation to disulfides under ambient conditions; the supply of too much oxygen, however, inhibits SAM formation. The electron transfer kinetics of the Si-S-enabled SAMs on Si-H is comparable to that on Au, suggesting that Si-S contacts are electrically transmissive. We further demonstrate the prospect of this spontaneous disulfide reduction by forming a monolayer of protein azurin on a Si-H surface within 1 h. The direct reduction of disulfides on Si electrodes presents new capabilities for a range of fields, including molecular electronics, for which highly conducting SAM-electrode contacts are necessary and for emerging fields such as biomolecular electronics as disulfide linkages could be exploited to wire proteins between Si electrodes, within the context of the current Si-based technologies.

On the Interface Design of Si and Multilayer Graphene for a High-Performance Li-Ion Battery Anode

Si/multilayer graphene (mG) is a promising candidate for the next-generation Li-ion battery anode. The highly ordered mG shows intrinsic good stability against the liquid electrolyte and its flexibility to accommodate volume change. Until now, reducing the growth temperature and thus engineering the interphase are a very important research area, but only few studies have been reported. Herein, for the first time, the mG is grown with the Al₂O₃ catalyst at a relatively low temperature of 750 °C, while the thickness is controlled to 2 nm. The growth of mG obeys the Stranski-Krastanov mechanism. Applying a rapid cooling process, a silicon oxycarbide (SiOC) interlayer is in situ-fabricated between the mG coating layer and Si core. The SiOC interlayer is demonstrated to accommodate the volume change of Si and enable faster lithium ion transportation than mG. Taking synergetic advantages of the mG coating layer and SiOC interphase, the cycling stability significantly improved, and a high specific capacity of 990 mA h/g is obtained at 1 A/g after 500 cycles in half cells. A high rate performance of 1164.5 mA h/g at 4 A/g is achieved. Tested in a 1.8 A h pouch cell with LiNi_0.5Mn_0.3Co_0.2O₂ (NMC532) as the cathode, the cell delivers a specific energy of ∼380 W h/kg. The capacity retentions are 93% and 78% after 100 cycles and 200 cycles, respectively. Our work highlights the importance of the interphase design of Si/mG composite anodes, which could also be extended to various core-shell materials in energy storage materials.

A thick yet dense silicon anode with enhanced interface stability in lithium storage evidenced by in situ TEM observations

Increasing the density and thickness of electrodes is required to maximize the volumetric energy density of lithium-ion batteries for practical applications. However, dense and thick electrodes, especially high-mass-content (>50 wt%) silicon anodes, have poor mechanical stability due to the presence of a large number of unstable interfaces between the silicon and conducting components during cycling. Here we report a network of mechanically robust carbon cages produced by the capillary shrinkage of graphene hydrogels that can contain the silicon nanoparticles in the cages and stabilize the silicon/carbon interfaces. In situ transmission electron microscope characterizations including compression and tearing of the structure and lithiation-induced silicon expansion experiments, have provided insight into the excellent confinement and buffering ability of this interface-strengthened graphene-caged silicon nanoparticle anode material. Consequently, a dense and thick silicon anode with reduced thickness fluctuations has been shown to deliver both high volumetric (>1000 mAh cm^-3) and areal (>6 mAh cm^-2) capacities together with excellent cycling capability.

Russian doll architecture enables a high-rate and long-life MnCo2O4/C-lithium battery

Realizing high capacity at high current densities is one of the challenges for battery electrode materials towards practical applications, especially for metal oxide electrode materials. Designing a specific structure that can alleviate volume expansion and accelerate the diffusion of the ions is an effective way to achieve this goal. Herein, a porous multilayer core-shell structured manganese cobalt oxide/carbon composite (MnCo₂O₄/C) was obtained by using a simple route that combines the hydrothermal method with calcination. The structure is similar to a Russian doll, which is nested with three to four layers of concentric porous shells. The porous multilayer core-shell structures can relieve volume expansion during discharge/charge and reduce the Li-ion diffusion path. Additionally, it can provide a richer activity site, thereby storing more lithium ions. When used as an anode material, the synthesized MnCo₂O₄/C showed a high specific capacity of 978 mAh g^-1 after 800 cycles at a current density of 1 A g^{- 1}. Even at a high current density of 10 A g^-1, the electrode could still deliver a specific capacity of 251 mAh g^-1, which makes it more suitable for powering large equipment such as electric vehicles.

Stable high-capacity and high-rate silicon-based lithium battery anodes upon two-dimensional covalent encapsulation

Silicon is a promising anode material for lithium-ion and post lithium-ion batteries but suffers from a large volume change upon lithiation and delithiation. The resulting instabilities of bulk and interfacial structures severely hamper performance and obstruct practical use. Stability improvements have been achieved, although at the expense of rate capability. Herein, a protocol is developed which we describe as two-dimensional covalent encapsulation. Two-dimensional, covalently bound silicon-carbon hybrids serve as proof-of-concept of a new material design. Their high reversibility, capacity and rate capability furnish a remarkable level of integrated performances when referred to weight, volume and area. Different from existing strategies, the two-dimensional covalent binding creates a robust and efficient contact between the silicon and electrically conductive media, enabling stable and fast electron, as well as ion, transport from and to silicon. As evidenced by interfacial morphology and chemical composition, this design profoundly changes the interface between silicon and the electrolyte, securing the as-created contact to persist upon cycling. Combined with a simple, facile and scalable manufacturing process, this study opens a new avenue to stabilize silicon without sacrificing other device parameters. The results hold great promise for both further rational improvement and mass production of advanced energy storage materials.

Stabilizing silicon without sacrificing other device parameters is essential for practical use in lithium and post lithium battery anodes. Here, the authors show the skin-like two-dimensional covalent encapsulation furnishing a remarkable level of integrated lithium storage performances of silicon.

Tailoring the Vertical Morphology of Organic Films for Efficient Planar-Si/Organic Hybrid Solar Cells by Facile Nonpolar Solvent Treatment

The optical and electrical properties of the blending organic film poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS) are strongly affected by its morphology, resulting in the performance variation in Si/organic hybrid solar cells. Here, a facile postsolvent treatment is used to tailor the vertical morphology of PEDOT:PSS by introducing a nonpolar solvent. X-ray photoelectron spectroscopy depth-profiling measurements show that the distribution of PEDOT and PSS on the surface of n-type Si can be changed by nonpolar solvent n-hexane (NHX) treatment, where more PSS aggregate at the bottom of the blend film and more PEDOT float up to the top, as compared with the reference sample. As a result, after NHX treatment, the average lifetime of the Si/organic films is increased from 152 μs for untreated samples to 248 μs for NHX-treated ones because of the better passivation effect of PSS on Si. Moreover, the transmission line model measurements indicate that the contact resistance (R_C) of PEDOT:PSS film and the Ag electrode is decreased for better charge collection after NHX treatment. Eventually, the best power conversion efficiency (PCE) of 13.78% for NHX-treated planar solar cells is obtained, much higher than the PCE (with best of 12.78%) of reference devices without nonpolar solvent treatment. Our results provide a facile method to tailor the vertical morphology of the PEDOT:PSS in Si/organic hybrid solar cells.

Spontaneous S-Si bonding of alkanethiols to Si(111)-H: towards Si-molecule-Si circuits

We report the synthesis of covalently linked self-assembled monolayers (SAMs) on silicon surfaces, using mild conditions, in a way that is compatible with silicon-electronics fabrication technologies. In molecular electronics, SAMs of functional molecules tethered to gold via sulfur linkages dominate, but these devices are not robust in design and not amenable to scalable manufacture. Whereas covalent bonding to silicon has long been recognized as an attractive alternative, only formation processes involving high temperature and/or pressure, strong chemicals, or irradiation are known. To make molecular devices on silicon under mild conditions with properties reminiscent of Au–S ones, we exploit the susceptibility of thiols to oxidation by dissolved O₂, initiating free-radical polymerization mechanisms without causing oxidative damage to the surface. Without thiols present, dissolved O₂ would normally oxidize the silicon and hence reaction conditions such as these have been strenuously avoided in the past. The surface coverage on Si(111)–H is measured to be very high, 75% of a full monolayer, with density-functional theory calculations used to profile spontaneous reaction mechanisms. The impact of the Si–S chemistry in single-molecule electronics is demonstrated using STM-junction approaches by forming Si–hexanedithiol–Si junctions. Si–S contacts result in single-molecule wires that are mechanically stable, with an average lifetime at room temperature of 2.7 s, which is five folds higher than that reported for conventional molecular junctions formed between gold electrodes. The enhanced “ON” lifetime of this single-molecule circuit enables previously inaccessible electrical measurements on single molecules.

Spontaneously formed Si–S bonds enable monolayer and single-molecule Si–molecule–Si circuits.

Interface Heteroatom-doping: Emerging Solutions to Silicon-based Anodes

Silicon-based composites have been recognized as a promising anode material for high-energy lithium-ion batteries (LIBs). However, the intrinsically low conductivity and the huge volume expansion during lithiation/delithiation progresses impede its further practical applications. In the past decades, numerous efforts have been made for surface and interface modification of Si-based anodes. Among these, doping of active materials with heteroatoms is one promising method to endow silicon many unmatched electrochemical properties. In this review, we focus on the effects of heteroatom doping on the interfacial properties of Si-based anodes, and some typical strategies for the interface doping are highlighted. We aim to give some reference for interfacial doping of Si-based anodes in LIBs.

Sandwich-Like C@SnS@TiO2 Anodes with High Power and Long Cycle for Li-Ion Storage

Up to now, high energy density batteries can be easily achieved by using alloys or conversion materials with high theoretical capacities (such as silicon-based and tin-based materials). However, these anode materials tend to sacrifice power densities while maintaining high energy densities. Herein, a sandwich-like C@SnS@TiO₂ anode with both high capacity and high power is designed by controlling a close integration between interfacial layers. The volume expansion of the middle layer of the SnS in the C@SnS@TiO₂ anode is greatly constrained by a synergetic interaction of the TiO₂ core and the carbon shell. From the results of the real-time dynamic evolution of electrode thickness during charging and discharging processes, the sandwich-like C@0.5SnS@TiO₂ has a max expansion rate of 11.5% in the first lithiation, which is much lower than that of pristine SnS (89.2%), and the expansion of C@0.5SnS@TiO₂ is basically reversible in the following charging/discharging processes. As a result, the sandwich-like C@0.5SnS@TiO₂ anode delivers a stable capacity of 660mAh g^-1 at 50 mA g^-1 and manifests an excellent rate capability, with a capacity of 357.2 mAh g^-1 at 5A g^-1 and a recovery ability of nearly 100%. In addition, it exhibits an outstanding long lifespan, retaining 95.6% capacity after 2500 cycles at 1A g^-1. This work presents a durable tin-based anode with moderate capacity for high-energy batteries and offers some ideas for the delicate study of materials with severe expansion during circulation.

Supremely elastic gel polymer electrolyte enables a reliable electrode structure for silicon-based anodes

Silicon-based materials are promising anodes for next-generation lithium-ion batteries, owing to their high specific capacities. However, the huge volume expansion and shrinkage during cycling result in severe displacement of silicon particles and structural collapse of electrodes. Here we report the use of a supremely elastic gel polymer electrolyte to address this problem and realize long-term stable cycling of silicon monoxide anodes. The high elasticity of the gel polymer electrolyte is attributed to the use of a unique copolymer consisting of a soft ether domain and a hard cyclic ring domain. Consequently, the displacement of silicon monoxide particles and volume expansion of the electrode were effectively reduced, and the damage caused by electrode cracking is alleviated. A SiO|LiNi0.5Co0.2Mn0.3O2 cell shows 70.0% capacity retention in 350 cycles with a commercial-level reversible capacity of 3.0 mAh cm-2 and an average Coulombic efficiency of 99.9%.

A hierarchical layering design for stable, self-restrained and high volumetric binder-free lithium storage

A hierarchical layering strategy is developed for silicon anodes. The resultant parallelly oriented graphene-sandwiched layered silicon/graphene hybrid microparticles exhibit stable cycling with high volumetric capacity when being charged and discharged at high rates and commercial loading levels, attributable to the designed architecture.

Largely Improved Battery Performance Using a Microsized Silicon Skeleton Caged by Polypyrrole as Anode

Various architectures with nanostructured silicon have demonstrated promising battery performance while posing a challenge in industrial production. The current ratio of silicon in graphite as anode is less than 5 wt %, which greatly limits the battery energy density. In this article, we report a scalable synthesis of a large silicon cage composite (micrometers) that is composed of a silicon skeleton and an ultrathin (<5 nm) mesoporous polypyrrole (PPy) skin via a facile wet-chemical method. The industry available, microsized AlSi alloy was used as precursor. The hollow skeleton configuration provides sufficient space to accommodate the drastic volume expansion/shrinkage upon charging/discharging, while the conductive polymer serves as a protective layer and fast channel for Li⁺/e^- transport. The battery with the microsilicon (μ-Si) cage as anode displays an excellent capacity retention upon long cycling at high charge/discharge rates and high material loadings. At 0.2 C, a specific capacity of ∼1660 mAh/g with a Coulombic efficiency (CE) of ∼99.8% and 99.4% was achieved after 500 cycles at 3 mg/cm² loading and 400 cycles at 4.4 mg/cm² loading, respectively. At 1.0 C, a capacity as high as 1149 mAh/g was retained after 500 cycles with such high silicon loading. The areal capacity of as high as 6.4 mAh/cm² with 4.4 mg/cm² loading was obtained, which ensures a high battery energy density in powering large devices such as electric vehicles.

Pd-Ag Alloy Electrocatalysts for CO2 Reduction: Composition Tuning to Break the Scaling Relationship

Constructing solid-solution-alloy electrocatalysts with tunable surface electronic configurations is the key to optimize intermediate bindings and thereby to promote the activity and selectivity of the CO₂ reduction reaction (CO₂RR). Herein, Pd_1-xAg_x alloy electrocatalysts are investigated as a platform to uncover the electronic effects on the CO₂RR. The optimal Pd_0.75Ag_0.25/C affords a superior CO Faradaic efficiency of 95.3% at -0.6 V (vs RHE) in 0.5 M KHCO₃, performing at a high level among recently reported electrocatalysts. Experimental and theoretical analysis further evidence that varying the composition of Pd_1-xAg_x alloys can effectively alter the electronic configurations and consequently break the inherent scaling relationship of the binding energy of different intermediates (*COOH and *CO). Among Pd_1-xAg_x, Pd_0.75Ag_0.25 gains the obviously weakened *CO and *H bindings but retained well the binding with *COOH, contributing to the facilitated kinetics toward CO product. Elucidating a feasible way to break the scaling relationship and further uncover the underlying mechanism, this work will inspire new design strategies toward active and selective electrocatalysts.

Hollow Carbon Nanobelts Codoped with Nitrogen and Sulfur via a Self-Templated Method for a High-Performance Sodium-Ion Capacitor

Sodium-ion capacitors (SICs) have attracted enormous attention due to their high energy density and high power density. In this work, N and S codoped hollow carbon nanobelts (N/S-HCNs) are synthesized by a self-templated method. The as-synthesized carbon nanobelts exhibit excellent performance in pseudocapacitance and electric double layer anions adsorption. After pairing the N/S-HCNs cathode with a tin foil anode in a carbonate electrolyte, the obtained SIC achieves a high specific capacity of 400 mAh g^-1 at 1 A g^-1 (based on the mass of cathode material) and energy density of 250.35 Wh kg^-1 at 676 W kg^-1 (based on the total mass of cathode and anode materials). Besides, the presented SIC also demonstrates high cycling stability with almost 100% capacity retention after 10 000 cycles, which is among the best results of the reported SICs, suggesting the potential for high-performance energy storage applications.

Spatially Resolving Lithiation in Silicon-Graphite Composite Electrodes via in Situ High-Energy X-ray Diffraction Computed Tomography

Optimizing the chemical and morphological parameters of lithium-ion (Li-ion) electrodes is extremely challenging, due in part to the absence of techniques to construct spatial and temporal descriptions of chemical and morphological heterogeneities. We present the first demonstration of combined high-speed X-ray diffraction (XRD) and XRD computed tomography (XRD-CT) to probe, in 3D, crystallographic heterogeneities within Li-ion electrodes with a spatial resolution of 1 μm. The local charge-transfer mechanism within and between individual particles was investigated in a silicon(Si)-graphite composite electrode. High-speed XRD revealed charge balancing kinetics between the graphite and Si during the minutes following the transition from operation to open circuit. Subparticle lithiation heterogeneities in both Si and graphite were observed using XRD-CT, where the core and shell structures were segmented, and their respective diffraction patterns were characterized.

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.

Fabrication and understanding of Cu3Si-Si@carbon@graphene nanocomposites as high-performance anodes for lithium-ion batteries

Besides silicon's low electronic conductivity, another critical issue for using silicon as the anode for lithium-ion batteries (LIBs) is the dramatic volume variation (>300%) during lithiation/delithiation processes, which can lead to rapid capacity fading and poor rate capability, thereby hampering silicon's practical applications in batteries. To mitigate these issues, herein, we report our findings on the design and understanding of a self-supported Cu₃Si-Si@carbon@graphene (Cu₃Si-SCG) nanocomposite anode. The nanocomposite is composed of Cu₃Si-Si core and carbon shell with core/shell particles uniformly encapsulated by graphene nanosheets anchored directly on a Cu foil. In this design, the carbon shell, the highly elastic graphene nanosheet, and the formed conductive and inactive Cu₃Si phase in Si serve as buffer media to suppress volume variation of Si during lithiation/delithiation processes and to facilitate the formation of a stable solid electrolyte interface (SEI) layer as well as to enable good transport kinetics. Chemomechanical simulation results quantitatively coincide with the in situ TEM observations of volume expansion and provide process details not seen in experiments. The optimized Cu₃Si-SCG nanocomposite anode exhibits good rate performance and delivers reversible capacity of 483 mA h g^-1 (based on the total weight of Cu₃Si-SCG) after 500 cycles with capacity retention of about 80% at high current density of 4 A g^-1, rendering the nanocomposite a desirable anode candidate for high-performance LIBs.