Deformations in Si-Li anodes upon electrochemical alloying in nano-confined space

Morphological Evolution of Sn-Metal-Based Anodes for Lithium-Ion Batteries Using Operando X-Ray Imaging

Sn-based electrodes are promising candidates for next-generation lithium-ion batteries. However, it suffers from deleterious micro-structural deformation as it undergoes drastic volume changes upon lithium insertion and extraction. Progress in designing these materials is limited to complex structures. There is a significant need to develop an alloy-based anode that can be industrially manufactured and offers high reversible capacity. This necessitates a profound understanding of the interplay between structural changes and electrochemical performance. Here, operando X-ray imaging is used to correlate the morphological evolution to electrochemical performance in foil and foam systems. The 3D Sn-foam-like structure electrode is fabricated in-house as a practical approach to accommodate the volume expansion and alleviate the mechanical stress experienced upon alloying/dealloying. Results show that generating pores in Sn electrodes can help manage the volume expansion and mitigate the severe mechanical stress in thick electrodes during alloying/dealloying processes. The foam electrode demonstrates superior electrochemical performance compared to non-porous Sn foil with an equivalent absolute capacity. This work advances the understanding of the real-time morphological evolution of Sn bulky electrodes.

Electrochemo-Mechanical Degradation and Failure of Active Particles in High Energy Density Batteries: A Review

Failure of the active particles is inherently electrochemo-mechanics dominated. This review comprehensively examines the electrochemo-mechanical degradation and failure mechanisms of active particles in high-energy density lithium-ion batteries. The study delves into the growth of passivating layers, such as the solid electrolyte interphase (SEI), and their impact on battery performance. It highlights the role of elevated temperatures in accelerating degradation reactions, such as the dissolution of transition metals and the formation of new SEI layers, leading to capacity fade and increased internal resistance. The review also discusses the mechanical degradation of electrode materials, including the fracture of active particles and the impact of stress on electrode performance. Advanced characterization techniques, such as cryogenic scanning transmission electron microscopy and 3D tomography, are explored to provide insights into the structural and chemical evolution of battery materials. By addressing the interplay between chemical, mechanical, and thermal factors, this review aims to provide guidelines for the chemistry development, material selection, structural design as well as recycling of next-generation batteries with high safety, durability, and high energy density.

Comparison Between Crystalline and Amorphous Silicon as Anodes for Lithium Ion Batteries: Electrochemical Performance from Practical Cells and Lithiation Behavior from Molecular Dynamics Simulations

As a prominent next-generation anode material for high-capacity applications, silicon stands out due to its potential. Crystalline silicon, which offers a higher initial capacity compared to its amorphous counterpart, presents challenges in practical applications due to its poor cycling performance. In this study, we prepared composites of crystalline and amorphous silicon with graphite, assembled pouch-type full cells, and evaluated their suitability for practical use. The material incorporating amorphous silicon demonstrated superior performance at both high and low rates, as well as various temperatures. Additionally, the changes in cell thickness during charge and discharge, i.e., the volume changes in the anode material, are significantly related to cycling performance. We examined the microscopic interactions between silicon and lithium atoms using molecular dynamics simulations. Our observations indicate that lithium migration within amorphous silicon, which has lower activation energy, is much easier than in crystalline silicon. In crystalline silicon, lithium penetration is greatly influenced by the orientation of the crystal planes, resulting in anisotropic volume expansion during lithiation.

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

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

Constructing a Stable Conductive Network for High-Performance Silicon-Based Anode in Lithium-Ion Batteries

The application of carbon nanotubes to silicon nanoparticles has been used to improve the electrical conductivity of silicon-carbon anodes and prevent agglomeration of silicon nanoparticles during cycling. In this study, the composites are synthesized through an uncomplicated technique that involves the ultrasonication mixing of pyrene derivatives and carbon nanotubes and the formation of complexes with silicon nanoparticles in ultrasonic dispersion and magnetic stirring and then treated under vacuum. When the prepared composites are applied as lithium-ion battery anodes, the Si@(POH-AOCNTs) electrode displays a high reversible capacity of 3254.7 mAh g-1 at a current density of 0.1 A g-1. Furthermore, it exhibits excellent cycling stability with a specific capacity of 1195.8 mAh g-1 after 500 cycles at 1.0 A g-1. The superior electrochemical performance may be attributed to a large π-conjugated electron system of pyrene derivatives, which prompts the formation of a homogeneous CNTs conductive network and ensures the effective electron transfer, while the interaction between hydroxyl functional groups of hydroxypyrene and binder synergizes with CNTs network to further enhance the cycling stability of the composite.

New Chemical Synthesis Strategy To Construct a Silicon/Carbon Nanotubes/Carbon-Integrated Composite with Outstanding Lithium Storage Capability

The Si/C anode is one of the most promising candidate materials for the next-generation lithium-ion batteries (LIBs). Herein, a silicon/carbon nanotubes/carbon (Si/CNTs/C) composite is in situ synthesized by a one-step reaction of magnesium silicide, calcium carbonate, and ferrocene. Transmission electron microscopy reveals that the growth of CNTs is attributed to the catalysis of iron atoms derived from the decomposition of ferrocene. In comparison to a Si/C composite, the cycle stability of the Si/CNTs/C composite can obviously be improved as an anode for LIBs. The enhanced performance is mainly attributed to the following factors: (i) the perfect combination of Si nanoparticles and in situ grown CNTs achieves high mechanical integrity and good electrical contact; (ii) Si nanoparticles are entangled in the CNT cage, effectively reducing the volume expansion upon cycling; and (iii) in situ grown CNTs can improve the conductivity of composites and provide lithium ion transport channels. Moreover, the full cell constructed by a LiFePO₄ cathode and Si/CNTs/C anode exhibits excellent cycling stability (137 mAh g^-1 after 300 cycles at 0.5 C with a capacity retention rate of 91.2%). This work provides a new way for the synthesis of a Si/C anode for high-performance LIBs.

Achieving Cycling Stability in Anode of Lithium-Ion Batteries with Silicon-Embedded Titanium Oxynitride Microsphere

Surface coating approaches for silicon (Si) have demonstrated potential for use as anodes in lithium-ion batteries (LIBs) to address the large volume change and low conductivity of Si. However, the practical application of these approaches remains a challenge because they do not effectively accommodate the pulverization of Si during cycling or require complex processes. Herein, Si-embedded titanium oxynitride (Si-TiON) was proposed and successfully fabricated using a spray-drying process. TiON can be uniformly coated on the Si surface via self-assembly, which can enhance the Si utilization and electrode stability. This is because TiON exhibits high mechanical strength and electrical conductivity, allowing it to act as a rigid and electrically conductive matrix. As a result, the Si-TiON electrodes delivered an initial reversible capacity of 1663 mA h g-1 with remarkably enhanced capacity retention and rate performance.

Mechanics-based design of lithium-ion batteries: a perspective

From the overall framework of battery development, the battery structures have not received enough attention compared to the chemical components in batteries. The mechanical-electrochemical coupling behavior is a starting point for investigation on battery structures and the subsequent battery design. This perspective systematically reviews the efforts on the mechanics-based design for lithium-ion batteries (LIBs). Two typical types of mechanics-based LIB designs, namely the design at the preparation stage and that at the cycling stage, have been discussed, respectively. The former systemizes the structure design of multiscale battery components from the particle level to the cell level. The latter focuses on the external mechanics-related control, including external pressures and charge-discharge protocols, of in-service LIBs. Moreover, the general problems currently being faced in the mechanics-based LIB design are summarized, followed by the outlook of possible solutions.

Quantifying the trade-off between stiffness and permeability in hydrogels

Hydrogels have a distinct combination of mechanical and water-transport behaviors. As hydrogels stiffen when they de-swell, they become less permeable. Here, we combine de Gennes' semi-dilute polymer theory with the Kozeny-Carman equation to develop a simple, succinct scaling law describing the relationship between mechanical stiffness and hydraulic permeability where permeability scales with stiffness to the -8/9 power. We find a remarkably close agreement between the scaling law and experimental results across four different polymer families with varied crosslinkings. This inverse relationship establishes a fundamental trade-off between permeability and stiffness.

Structure and reaction mechanism of binary Ni-Al oxides as materials for lithium-ion battery anodes

A nanometer sized solid solution of NiO and Al₂O₃ was synthesized by calcination of Ni-Al layered double hydroxides (LDHs). The crystal structure of the obtained compound was determined by XRD and XAFS analyses: Ni²⁺ and Al³⁺ ions are located at the metal ion site of the rock salt structure and a certain amount of cation vacancies are also introduced for charge compensation. The electrochemical properties of the Ni-Al binary metal oxide as an anode material for lithium ion batteries were examined by the constant current charge-discharge test. Ni-Al oxide showed higher charging capacity in comparison with pristine NiO. In particular, the capacity in the lower voltage region (below 1.5 V), the limited capacity in this region is the weak point of the conversion anode, was improved to 540 mA h g^-1 that is about twice that of pristine NiO. This improvement in the capacity in the lower voltage region is concluded to be due to the redox activity of Al³⁺ ions during the charge-discharge on the basis of the results of electrochemical measurements and ex situ XAFS measurements at the Ni and Al edge. The reaction mechanism of this compound is investigated using ex situ XRD and XAFS methods. For the charge (reduction) in the higher voltage region (OCV-1.0 V), lithium ion intercalation into the cation vacancy sites and/or lithium ion adsorption on the surface of particles are proceeding. For the charge in the lower voltage region (1.0-0.03 V), conversion reaction occurs by the reduction of Ni²⁺ and Al³⁺ ions to metal particles with surface electrolyte interface (SEI) layer formation. For the discharge in the lower voltage region (0.03-1.5 V), only Al metal particles are oxidized to Al³⁺ ions and some intermediate complexes are formed. For the discharge in the higher voltage region (1.5-3.0 V), the lattice of the Ni-Al binary oxide solid solution is reconstructed with the oxidation of Ni to Ni²⁺.

Molecular Understanding of Electrochemical-Mechanical Responses in Carbon-Coated Silicon Nanotubes during Lithiation

Carbon-coated silicon nanotube (SiNT@CNT) anodes show tremendous potential in high-performance lithium ion batteries (LIBs). Unfortunately, to realize the commercial application, it is still required to further optimize the structural design for better durability and safety. Here, the electrochemical and mechanical evolution in lithiated SiNT@CNT nanohybrids are investigated using large-scale atomistic simulations. More importantly, the lithiation responses of SiNW@CNT nanohybrids are also investigated in the same simulation conditions as references. The simulations quantitatively reveal that the inner hole of the SiNT alleviates the compressive stress concentration between a-Li_xSi and C phases, resulting in the SiNT@CNT having a higher Li capacity and faster lithiation rate than SiNW@CNT. The contact mode significantly regulates the stress distribution at the inner hole surface, further affecting the morphological evolution and structural stability. The inner hole of bare SiNT shows good structural stability due to no stress concentration, while that of concentric SiNT@CNT undergoes dramatic shrinkage due to compressive stress concentration, and that of eccentric SiNT@CNT is deformed due to the mismatch of stress distribution. These findings not only enrich the atomic understanding of the electrochemical–mechanical coupled mechanism in lithiated SiNT@CNT nanohybrids but also provide feasible solutions to optimize the charging strategy and tune the nanostructure of SiNT-based electrode materials.

Carbon/Polymer Bilayer-Coated Si-SiOx Electrodes with Enhanced Electrical Conductivity and Structural Stability

Si-based electrodes offer exceptionally high capacity and energy density for lithium-ion batteries (LIBs),but suffer from poor structural stability and electrical conductivity that hamper their practical applications. To tackle these obstacles, we design a C/polymer bilayer coating deposited on Si-SiO_x microparticles. The inner C coating is used to improve electrical conductivity. The outer C-nanoparticle-reinforced polypyrrole (CNP-PPy) is a polymer matrix composite that can minimize the volumetric expansion of Si-SiO_x and enhance its structural stability during battery operation. Electrodes made of such robust Si-SiO_x@C/CNP-PPy microparticles exhibit excellent cycling performance: 83% capacity retention (794 mAh g^-1) at a 2 C rate after more than 900 cycles for a coin-type half cell, and 80% capacity retention (with initial energy density of 308 Wh kg^-1) after over 1100 cycles for a pouch-type full cell. By comparing the samples with different coatings, an in-depth understanding of the performance enhancement is achieved, i.e., the C/CNP-PPy with cross-link bondings formed in the bilayer coating plays a key role for the improved structural stability. Moreover, a full battery using the Si-SiO_x@C/CNP-PPy electrode successfully drives a car model, demonstrating a bright application prospect of the C/polymer bilayer coating strategy to make future commercial LIBs with high stability and energy density.

Circumventing huge volume strain in alloy anodes of lithium batteries

Since the launch of lithium-ion batteries, elements (such as silicon, tin, or aluminum) that can be alloyed with lithium have been expected as anode materials, owing to larger capacity. However, their successful application has not been accomplished because of drastic structural degradation caused by cyclic large volume change during battery reactions. To prolong lifetime of alloy anodes, we must circumvent the huge volume strain accompanied by insertion/extraction of lithium. Here we report that by using aluminum-foil anodes, the volume expansion during lithiation can be confined to the normal direction to the foil and, consequently, the electrode cyclability can be markedly enhanced. Such a unidirectional volume-strain circumvention requires an appropriate hardness of the matrix and a certain tolerance to off-stoichiometry of the resulting intermetallic compound, which drive interdiffusion of matrix component and lithium along the normal-plane direction. This metallurgical concept would invoke a paradigm shift to future alloy-anode battery technologies.

Alloy anode materials in lithium batteries usually suffer from fatal structural degradation due to the large volume change during cycling. Here the authors report a design in which Al foil serves as both anode and current collector to circumvent the strain.

Chemical reactivity under nanoconfinement

Confining molecules can fundamentally change their chemical and physical properties. Confinement effects are considered instrumental at various stages of the origins of life, and life continues to rely on layers of compartmentalization to maintain an out-of-equilibrium state and efficiently synthesize complex biomolecules under mild conditions. As interest in synthetic confined systems grows, we are realizing that the principles governing reactivity under confinement are the same in abiological systems as they are in nature. In this Review, we categorize the ways in which nanoconfinement effects impact chemical reactivity in synthetic systems. Under nanoconfinement, chemical properties can be modulated to increase reaction rates, enhance selectivity and stabilize reactive species. Confinement effects also lead to changes in physical properties. The fluorescence of light emitters, the colours of dyes and electronic communication between electroactive species can all be tuned under confinement. Within each of these categories, we elucidate design principles and strategies that are widely applicable across a range of confined systems, specifically highlighting examples of different nanocompartments that influence reactivity in similar ways.

Multiscale Multiphase Lithiation and Delithiation Mechanisms in a Composite Electrode Unraveled by Simultaneous Operando Small-Angle and Wide-Angle X-Ray Scattering

The (de)lithiation process and resulting atomic and nanoscale morphological changes of an a-Si/c-FeSi₂/graphite composite negative electrode are investigated within a Li-ion full cell at several current rates (C-rates) and after prolonged cycling by simultaneous operando synchrotron wide-angle and small-angle X-ray scattering (WAXS and SAXS). WAXS allows the probing of the local crystalline structure. In particular, the observation of the graphite (de)lithiation process, revealed by the Li_xC₆ Bragg reflections, enables access to the respective capacities of both graphite and active silicon. Simultaneously and independently, information on the silicon state of (de)lithiation and nanoscale morphology (1 to 60 nm) is obtained through SAXS. During lithiation, the SAXS intensity in the region corresponding to characteristic distances within the a-Si/c-FeSi₂ domains increases. The combination of the SAXS/WAXS measurements over the course of several charge/discharge cycles, in pristine and aged electrodes, provides a complete picture of the C-rate-dependent sequential (de)lithiation mechanism of the a-Si/c-FeSi₂/graphite anode. Our results indicate that, within the composite electrode, the active silicon volume does not increase linearly with lithium insertion and point toward the important role of the electrode morphology to accommodate the nanoscale silicon expansion, an effect that remains beneficial after cell aging and most probably explains the excellent performance of the composite material.

Multiscale Buffering Engineering in Silicon-Carbon Anode for Ultrastable Li-Ion Storage

Silicon-carbon (Si-C) hybrids have been proven to be the most promising anodes for the next-generation lithium-ion batteries (LIBs) due to their superior theoretical capacity (∼4200 mAh g^-1). However, it is still a critical challenge to apply this material for commercial LIB anodes because of the large volume expansion of Si, unstable solid-state interphase (SEI) layers, and huge internal stresses upon lithiation/delithiation. Here, we propose an engineering concept of multiscale buffering, taking advantage of a nanosized Si-C nanowire architecture through fabricating specific microsized wool-ball frameworks to solve all the above-mentioned problems. These wool-ball-like frameworks, prepared at high yields, nearly matching industrial scales (they can be routinely produced at a rate of ∼300 g/h), are composed of Si/C nanowire building blocks. As anodes, the Si-C wool-ball frameworks show ultrastable Li⁺ storage (2000 mAh g^-1 for 1000 cycles), high initial Coulombic efficiency of ∼90%, and volumetric capacity of 1338 mAh cm^-3. In situ TEM proves that the multiscale buffering design enables a small volume variation, only ∼19.5%, reduces the inner stresses, and creates a very thin SEI. The perfect multiscale elastic buffering makes this material more stable compared to common Si nanoparticle-assembled counterpart electrodes.

Phase evolution of conversion-type electrode for lithium ion batteries

Batteries with conversion-type electrodes exhibit higher energy storage density but suffer much severer capacity fading than those with the intercalation-type electrodes. The capacity fading has been considered as the result of contact failure between the active material and the current collector, or the breakdown of solid electrolyte interphase layer. Here, using a combination of synchrotron X-ray absorption spectroscopy and in situ transmission electron microscopy, we investigate the capacity fading issue of conversion-type materials by studying phase evolution of iron oxide composited structure during later-stage cycles, which is found completely different from its initial lithiation. The accumulative internal passivation phase and the surface layer over cycling enforce a rate−limiting diffusion barrier for the electron transport, which is responsible for the capacity degradation and poor rate capability. This work directly links the performance with the microscopic phase evolution in cycled electrode materials and provides insights into designing conversion-type electrode materials for applications.

Conversion electrodes possess high energy density but suffer a rapid capacity loss over cycling compared to their intercalation equivalents. Here the authors reveal the microscopic origin of the fading behavior, showing that the formation and augmentation of passivation layers are responsible.

POSS-Derived Synthesis and Full Life Structural Analysis of Si@C as Anode Material in Lithium Ion Battery

Polyhedral oligomeric silsesquioxane (POSS)-derived Si@C anode material is prepared by the copolymerization of octavinyl-polyhedral oligomeric silsesquioxane (octavinyl-POSS) and styrene. Octavinyl-polyhedral oligomeric silsesquioxane has an inorganic core (-Si₈O12) and an organic vinyl shell. Carbonization of the core-shell structured organic-inorganic hybrid precursor results in the formation of carbon protected Si-based anode material applicable for lithium ion battery. The initial discharge capacity of the battery based on the as-obtained Si@C material Si reaches 1500 mAh g-1. After 550 charge-discharge cycles, a high capacity of 1430 mAh g-1 was maintained. A combined XRD, XPS and TEM analysis was performed to investigate the variation of the discharge performance during the cycling experiments. The results show that the decrease in discharge capacity in the first few cycles is related to the formation of solid electrolyte interphase (SEI). The subsequent rise in the capacity can be ascribed to the gradual morphology evolution of the anode material and the loss of capacity after long-term cycles is due to the structural pulverization of silicon within the electrode. Our results not only show the high potential of the novel electrode material but also provide insight into the dynamic features of the material during battery cycling, which is useful for the future design of high-performance electrode material.

Formation of Si Hollow Structures as Promising Anode Materials through Reduction of Silica in AlCl3-NaCl Molten Salt

Hollow nanostructures are attractive for energy storage and conversion, drug delivery, and catalysis applications. Although these hollow nanostructures of compounds can be generated through the processes involving the well-established Kirkendall effect or ion exchange method, a similar process for the synthesis of the pure-substance one ( e. g., Si) remains elusive. Inspired by the above two methods, we introduce a continuous ultrathin carbon layer on the silica nano/microstructures (Stöber spheres, diatom frustules, sphere in sphere) as the stable reaction interface. With the layer as the diffusion mediator of the reactants, silica structures are successfully reduced into their porous silicon hollow counterparts with metal Al powder in AlCl₃-NaCl molten salt. The structures are composed of silicon nanocrystallites with sizes of 15-25 nm. The formation mechanism can be explained as an etching-reduction/nucleation-growth process. When used as the anode material, the silicon hollow structure from diatom frustules delivers specific capacities of 2179, 1988, 1798, 1505, 1240, and 974 mA h g^-1 at 0.5, 1, 2, 4, 6, and 8 A g^-1, respectively. After being prelithiated, it retains 80% of the initial capacity after 1100 cycles at 8 A g^-1. This work provides a general way to synthesize versatile silicon hollow structures for high-performance lithium ion batteries due to the existence of ample silica reactants and can be extended to the synthesis of hollow structures of other materials.

Hierarchical Carbon-Coated Ball-Milled Silicon: Synthesis and Applications in Free-Standing Electrodes and High-Voltage Full Lithium-Ion Batteries

Lithium-ion batteries have been regarded as one of the most promising energy storage devices, and development of low-cost batteries with high energy density is highly desired so that the cost per watt-hour ($/Wh) can be minimized. In this work, we report using ball-milled low-cost silicon (Si) as the starting material and subsequent carbon coating to produce low-cost hierarchical carbon-coated (HCC) Si. The obtained particles prepared from different Si sources all show excellent cycling performance of over 1000 mAh/g after 1000 cycles. Interestingly, we observed in situ formation of porous Si, and it is well confined in the carbon shell based on postcycling characterization of the hierarchical carbon-coated metallurgical Si (HCC-M-Si) particles. In addition, lightweight and free-standing electrodes consisting of the HCC-M-Si particles and carbon nanofibers were fabricated, which achieved 1015 mAh/g after 100 cycles based on the total mass of the electrodes. Compared with conventional electrodes, the lightweight and free-standing electrodes significantly improve the energy density by 745%. Furthermore, LiCoO₂ and LiNi_0.5Mn_1.5O₄ cathodes were used to pair up with the HCC-M-Si anode to fabricate full cells. With LiNi_0.5Mn_1.5O₄ as cathode, an energy density up to 547 Wh/kg was achieved by the high-voltage full cell. After 100 cycles, the full cell with a LiNi_0.5Mn_1.5O₄ cathode delivers 46% more energy density than that of the full cell with a LiCoO₂ cathode. The systematic investigation on low-cost Si anodes together with their applications in lightweight free-standing electrodes and high-voltage full cells will shed light on the development of high-energy Si-based lithium-ion batteries for real applications.

Zeolite-Templated Mesoporous Silicon Particles for Advanced Lithium-Ion Battery Anodes

For the practical use of high-capacity silicon anodes in high-energy lithium-based batteries, key issues arising from the large volume change of silicon during cycling must be addressed by the facile structural design of silicon. Herein, we discuss the zeolite-templated magnesiothermic reduction synthesis of mesoporous silicon (mpSi) (mpSi-Y, -B, and -Z derived from commercial zeolite Y, Beta, and ZSM-5, respectively) microparticles having large pore volume (0.4-0.5 cm³/g), wide open pore size (19-31 nm), and small primary silicon particles (20-35 nm). With these appealing mpSi particle structural features, a series of mpSi/C composites exhibit outstanding performance including excellent cycling stabilities for 500 cycles, high specific and volumetric capacities (1100-1700 mAh g^-1 and 640-1000 mAh cm^-3 at 100 mA g^-1), high Coulombic efficiencies (approximately 100%), and remarkable rate capabilities, whereas conventional silicon nanoparticles (SiNP)/C demonstrate limited cycle life. These enhanced electrochemical responses of mpSi/C composites are further manifested by low impedance build-up, high Li ion diffusion rate, and small electrode thickness changes after cycling compared with those of SiNP/C composite. In addition to the outstanding electrochemical properties, the low-cost materials and high-yield processing make the mpSi/C composites attractive candidates for high-performance and high-energy Li-ion battery anodes.

In situ analytical techniques for battery interface analysis

Interface is a key to high performance and safe lithium-ion batteries or lithium batteries.

Air-Stable Porous Fe2N Encapsulated in Carbon Microboxes with High Volumetric Lithium Storage Capacity and a Long Cycle Life

The development of inexpensive electrode materials with a high volumetric capacity and long cycle-life is a central issue for large-scale lithium-ion batteries. Here, we report a nanostructured porous Fe₂N anode fully encapsulated in carbon microboxes (Fe₂N@C) prepared through a facile confined anion conversion from polymer coated Fe₂O₃ microcubes. The resulting carbon microboxes could not only protect the air-sensitive Fe₂N from oxidation but also retain thin and stable SEI layer. The appropriate internal voids in the Fe₂N cubes help to release the volume expansion during lithiation/delithiation processes, and Fe₂N is kept inside the carbon microboxes without breaking the shell, resulting in a very low electrode volume expansion (the electrode thickness variation upon lithiation is ∼9%). Therefore, the Fe₂N@C electrodes maintain high volumetric capacity (1030 mA h cm^-3 based on the lithiation-state electrode volume) comparable to silicon anodes, stable cycling performance (a capacity retention of over 91% for 2500 cycles), and excellent rate performance. Kinetic analysis reveals that the Fe₂N@C shows an enhanced contribution of capacitive charge mechanism and displays typical pseudocapacitive behavior. This work provides a new direction on designing and constructing nanostructured electrodes and protective layer for air unstable conversion materials for potential applications as a lithium-ion battery/capacitor electrode.

Study of Lithium Silicide Nanoparticles as Anode Materials for Advanced Lithium Ion Batteries

The development of high-performance silicon anodes for the next generation of lithium ion batteries (LIBs) evokes increasing interest in studying its lithiated counterpart-lithium silicide (Li_xSi). In this paper we report a systematic study of three thermodynamically stable phases of Li_xSi (x = 4.4, 3.75, and 2.33) plus nitride-protected Li_4.4Si, which are synthesized via the high-energy ball-milling technique. All three Li_xSi phases show improved performance over that of unmodified Si, where Li_4.4Si demonstrates optimum performance with a discharging capacity of 3306 (mA h)/g initially and maintains above 2100 (mA h)/g for over 30 cycles and above 1200 (mA h)/g for over 60 cycles at the current density of 358 mA/g of Si. A fundamental question studied is whether different electrochemical paradigms, that is, delithiation first or lithiation first, influence the electrode performance. No significant difference in electrode performance is observed. When a nitride layer (Li_xN_ySi_z) is created on the surface of Li_4.4Si, the cyclability is improved to retain the capacity above 1200 (mA h)/g for more than 80 cycles. By increasing the nitridation extent, the capacity retention is improved significantly from the average decrease of 1.06% per cycle to 0.15% per cycle, while the initial discharge capacity decreases due to the inactivity of Si in the Li_xN_ySi_z layer. Moreover, the Coulombic efficiencies of all Li_xSi-based electrodes in the first cycle are significantly higher than that of a Si electrode (∼90% vs 40-70%).

Nanospherical solid electrolyte interface layer formation in binder-free carbon nanotube aerogel/Si nanohybrids to provide lithium-ion battery anodes with a long-cycle life and high capacity

Silicon anodes for lithium ion batteries (LiBs) have been attracting considerable attention due to a theoretical capacity up to about 10 times higher than that of conventional graphite. However, huge volume expansion during the cycle causes cracks in the silicon, resulting in the degradation of cycling performance and eventual failure. Moreover, low electrical conductivity and an unstable solid electrolyte interface (SEI) layer resulting from repeated changes in volume still block the next step forward for the commercialization of the silicon material. Herein we demonstrate the carbon nanotube (CNT) aerogel/Si nanohybrid structure for anode materials of LiBs via freeze casting followed by an RF magnetron sputtering process, exhibiting improved capacity retention compared to Si only samples during 1000 electrochemical cycles. The CNT aerogels as 3D porous scaffold structures could provide buffer volume for the expansion/shrinkage of Si lattices upon cycling and increase electrical conductivity. In addition, the nanospherical and relatively thin SEI layers of the CNT aerogel/Si nanohybrid structure show better lithium ion diffusion characteristics during cycling. For this reason, the Si@CNT aerogel anode still yielded a high specific capacity of 1439 mA h g^-1 after 1000 charge/discharge cycles with low capacity fading. Our approach could be applied to other group IV LiB materials that undergo large volume changes, and also has promising potential for high performance energy applications.

Carbon-Coated Silicon Nanowires on Carbon Fabric as Self-Supported Electrodes for Flexible Lithium-Ion Batteries

A novel self-supported electrode with long cycling life and high mass loading was developed based on carbon-coated Si nanowires grown in situ on highly conductive and flexible carbon fabric substrates through a nickel-catalyzed one-pot atmospheric pressure chemical vapor deposition. The high-quality carbon coated Si nanowires resulted in high reversible specific capacity (∼3500 mA h g^-1 at 100 mA g^-1), while the three-dimensional electrode's unique architecture leads to a significantly improved robustness and a high degree of electrode stability. An exceptionally long cyclability with a capacity retention of ∼66% over 500 cycles at 1.0 A g^-1 was achieved. The controllable high mass loading enables an electrode with extremely high areal capacity of ∼5.0 mA h cm^-2. Such a scalable electrode fabrication technology and the high-performance electrodes hold great promise in future practical applications in high energy density lithium-ion batteries.

Advanced anodes composed of graphene encapsulated nano-silicon in a carbon nanotube network

In situ growth of hierarchical Gra/CNT was achieved for a Si@Gra@CNT composite, and the composite exhibit improved electrochemical performance as a LIB anode.

Flower-like carbon with embedded silicon nano particles as an anode material for Li-ion batteries

A novel 3-dimensional (3D) flower-like silicon/carbon composite was synthesized through spray drying method by using NaCl as the sacrificial reagent and was evaluated as an anode material for lithium ion batteries.

A micro-sized Si–CNT anode for practical application via a one-step, low-cost and green method

Silicon (Si) has been used in Li-ion batteries (LIBs), and considerable progress has been achieved in design and engineering with improved capacity and cycling.

The role of nanotechnology in the development of battery materials for electric vehicles

A significant amount of battery research and development is underway, both in academia and industry, to meet the demand for electric vehicle applications. When it comes to designing and fabricating electrode materials, nanotechnology-based approaches have demonstrated numerous benefits for improved energy and power density, cyclability and safety. In this Review, we offer an overview of nanostructured materials that are either already commercialized or close to commercialization for hybrid electric vehicle applications, as well as those under development with the potential to meet the requirements for long-range electric vehicles.

Synthesis and Electrochemical Lithium Storage Behavior of Carbon Nanotubes Filled with Iron Sulfide Nanoparticles

Carbon nanotubes (CNTs) filled with iron sulfide nanoparticles (NPs) are prepared by inserting sulfur and ferrocene into the hollow core of CNTs followed by heat treatment. It is found that pyrrhotite-11T iron sulfide (Fe-S) NPs with an average size of ≈15 nm are encapsulated in the tubular cavity of the CNTs (Fe-S@CNTs), and each particle is a single crystal. When used as the anode material of lithium-ion batteries, the Fe-S@CNT material exhibits excellent electrochemical lithium storage performance in terms of high reversible capacity, good cyclic stability, and desirable rate capability. In situ transmission electron microscopy studies show that the CNTs not only play an essential role in accommodating the volume expansion of the Fe-S NPs but also provide a fast transport path for Li ions. The results demonstrate that CNTs act as a unique nanocontainer and reactor that permit the loading and formation of electrochemically active materials with desirable electrochemical lithium storage performance. CNTs with their superior structural stability and Li-ion transfer kinetics are responsible for the improved rate capability and cycling performance of Fe-S NPs in CNTs.

In Situ and Ex Situ TEM Study of Lithiation Behaviours of Porous Silicon Nanostructures

In this work, we study the lithiation behaviours of both porous silicon (Si) nanoparticles and porous Si nanowires by in situ and ex situ transmission electron microscopy (TEM) and compare them with solid Si nanoparticles and nanowires. The in situ TEM observation reveals that the critical fracture diameter of porous Si particles reaches up to 1.52 μm, which is much larger than the previously reported 150 nm for crystalline Si nanoparticles and 870 nm for amorphous Si nanoparticles. After full lithiation, solid Si nanoparticles and nanowires transform to crystalline Li₁₅Si₄ phase while porous Si nanoparticles and nanowires transform to amorphous Li_xSi phase, which is due to the effect of domain size on the stability of Li₁₅Si₄ as revealed by the first-principle molecular dynamic simulation. Ex situ TEM characterization is conducted to further investigate the structural evolution of porous and solid Si nanoparticles during the cycling process, which confirms that the porous Si nanoparticles exhibit better capability to suppress pore evolution than solid Si nanoparticles. The investigation of structural evolution and phase transition of porous Si nanoparticles and nanowires during the lithiation process reveal that they are more desirable as lithium-ion battery anode materials than solid Si nanoparticles and nanowires.

In situ and operando atomic force microscopy of high-capacity nano-silicon based electrodes for lithium-ion batteries

Silicon is a promising next-generation anode material for high-energy-density lithium-ion batteries. While the alloying of nano- and micron size silicon with lithium is relatively well understood, the knowledge of mechanical degradation and structural rearrangements in practical silicon-based electrodes during operation is limited. Here, we demonstrate, for the first time, in situ and operando atomic force microscopy (AFM) of nano-silicon anodes containing polymer binder and carbon black additive. With the help of this technique, the surface topography is analyzed while electrochemical reactions are occurring. In particular, changes in particle size as well as electrode structure and height are visualized with high resolution. Furthermore, the formation and evolution of the solid-electrolyte interphase (SEI) can be followed and its thickness determined by phase imaging and nano-indentation, respectively. Major changes occur in the first lithiation cycle at potentials below 0.6 V with respect to Li/Li(+) due to increased SEI formation - which is a dynamic process - and alloying reactions. Overall, these results provide insight into the function of silicon-based composite electrodes and further show that AFM is a powerful technique that can be applied to important battery materials, without restriction to thin film geometries.

Theoretical Limits of Energy Density in Silicon-Carbon Composite Anode Based Lithium Ion Batteries

Silicon (Si) is under consideration as a potential next-generation anode material for the lithium ion battery (LIB). Experimental reports of up to 40% increase in energy density of Si anode based LIBs (Si-LIBs) have been reported in literature. However, this increase in energy density is achieved when the Si-LIB is allowed to swell (volumetrically expand) more than graphite based LIB (graphite-LIB) and beyond practical limits. The volume expansion of LIB electrodes should be negligible for applications such as automotive or mobile devices. We determine the theoretical bounds of Si composition in a Si–carbon composite (SCC) based anode to maximize the volumetric energy density of a LIB by constraining the external dimensions of the anode during charging. The porosity of the SCC anode is adjusted to accommodate the volume expansion during lithiation. The calculated threshold value of Si was then used to determine the possible volumetric energy densities of LIBs with SCC anode (SCC-LIBs) and the potential improvement over graphite-LIBs. The level of improvement in volumetric and gravimetric energy density of SCC-LIBs with constrained volume is predicted to be less than 10% to ensure the battery has similar power characteristics of graphite-LIBs.

Silicon oxycarbide glass-graphene composite paper electrode for long-cycle lithium-ion batteries

Silicon and graphene are promising anode materials for lithium-ion batteries because of their high theoretical capacity; however, low volumetric energy density, poor efficiency and instability in high loading electrodes limit their practical application. Here we report a large area (approximately 15 cm × 2.5 cm) self-standing anode material consisting of molecular precursor-derived silicon oxycarbide glass particles embedded in a chemically-modified reduced graphene oxide matrix. The porous reduced graphene oxide matrix serves as an effective electron conductor and current collector with a stable mechanical structure, and the amorphous silicon oxycarbide particles cycle lithium-ions with high Coulombic efficiency. The paper electrode (mass loading of 2 mg cm(-2)) delivers a charge capacity of ∼588 mAh g(-1)electrode (∼393 mAh cm(-3)electrode) at 1,020th cycle and shows no evidence of mechanical failure. Elimination of inactive ingredients such as metal current collector and polymeric binder reduces the total electrode weight and may provide the means to produce efficient lightweight batteries.

Bundle-like α'-NaV2O5 mesocrystals: from synthesis, growth mechanism to analysis of Na-ion intercalation/deintercalation abilities

Bundle-like α'-NaV2O5 mesocrystals were synthesized successfully by a two-step hydrothermal method. Observations using electron microscopy revealed that the obtained NaV2O5 mesocrystals were composed of nanobelts with the preferential growth direction of [010]. The precise crystal structure was further confirmed by Rietveld refinement and Raman spectroscopy. Based on analysis of crystal structure and microscopy, a reaction and growth mechanism, hydrolysis-condensation (oxolation and olation)-ion exchange-self-assembly, was proposed and described in detail. Furthermore, electrochemical measurements were used to analyze the Na-ions intercalation/deintercalation abilities in NaV2O5, and indicated that Na-ions were difficult to extract. Importantly, the DFT theoretical calculation results, which showed that the migration energy of Na-ions was so huge that migration of Na-ions was quite difficult, can explain and support well the results of the electrochemical measurements.

Enhanced Lithiation Cycle Stability of ALD-Coated Confined a-Si Microstructures Determined Using In Situ AFM

Microfabricated amorphous silicon (a-Si) pits ∼4 μm in diameter and 100 nm thick were fabricated to be partially confined in a nickel (Ni) current collector. Corresponding unconfined pillars were also fabricated. The samples were coated with 1.5, 3, or 6 nm of Al2O3 ALD. These samples were tested in electrolytes of 3:7 by weight ethylene carbonate:ethyl methyl carbonate (EC:EMC) with 1.2 M LiPF6 salt with and without 2% fluoroethylene carbonate (FEC) and in a pure FEC electrolyte with 10 wt % LiPF6. The samples were imaged with an atomic force microscope during electrochemical cycling to evaluate morphology evolution and solid electrolyte interphase (SEI) formation. The partially confined a-Si structures had superior cycle efficiency relative to the unconfined a-Si pillars. Additionally, samples with 3 nm of ALD achieved higher charge capacity and enhanced cycle life compared to samples without ALD, demonstrated thinner SEI formation, and after 10 cycles at a 1 C rate remained mostly intact and had actually decreased in diameter. Finally, the samples with 3 nm of ALD had better capacity retention in the baseline 3:7 EC:EMC than in either of the FEC containing electrolytes.

Atomistics of the lithiation of oxidized silicon (SiOx) nanowires in reactive molecular dynamics simulations

The atomistic lithiation mechanism of silicon oxides (SiO_x) is clarified using the ReaxFF reactive molecular dynamics simulation.

The importance of covalent coupling in the synthesis of high performance composite anodes for lithium ion batteries

We present a direct comparison between identical electrostatically and covalently assembled Si–graphene composites for lithium ion battery anodes.

Hybrid two-dimensional materials in rechargeable battery applications and their microscopic mechanisms

Advances in two-dimensional (2D) hybrid nanomaterials in electrochemical energy storage and their microscopic mechanisms are summarized and reviewed.

Challenges in Accommodating Volume Change of Si Anodes for Li-Ion Batteries

Si has been considered as a promising alternative anode for next-generation Li-ion batteries (LIBs) because of its high theoretical energy density, relatively low working potential, and abundance in nature. However, Si anodes exhibit rapid capacity decay and an increase in the internal resistance, which are caused by the large volume changes upon Li insertion and extraction. This unfortunately limits their practical applications. Therefore, managing the total volume change remains a critical challenge for effectively alleviating the mechanical fractures and instability of solid-electrolyte-interphase products. In this regard, we review the recent progress in volume-change-accommodating Si electrodes and investigate their ingenious structures with significant improvements in the battery performance, including size-controlled materials, patterned thin films, porous structures, shape-preserving shell designs, and graphene composites. These representative approaches potentially overcome the large morphologic changes in the volume of Si anodes by securing the strain relaxation and structural integrity in the entire electrode. Finally, we propose perspectives and future challenges to realize the practical application of Si anodes in LIB systems.

Hollow Structured Silicon Anodes with Stabilized Solid Electrolyte Interphase Film for Lithium-Ion Batteries

Silicon has been considered as a promising anode material for the next generation of lithium-ion batteries due to its high specific capacity. Its huge volume expansion during the alloying reaction with lithium spoils the stability of the interface between electrode and electrolyte, resulting in capacity degradation. Herein, we synthesized a novel hollow structured silicon material with interior space for accumulating the volume change during the lithiation. The as-prepared material shows excellent cycling stability, with a reversible capacity of ∼1650 m Ah g(-1) after 100 cycles, corresponding to 92% retention. The electrochemical impedance spectroscopy and differential scanning calorimetry were carried out to monitor the growth of SEI film, and the results confirm the stable solid electrolyte interphase film on the surface of hollow structured silicon.