Structural and chemical interplay between nano-active and encapsulation materials in a core–shell SnO2@MXene lithium ion anode system

Structural and chemical interplay between nano-active and encapsulation materials in a core–shell SnO₂@MXene lithium ion anode system was investigated in detail.

Development of in situ optical spectroscopy with high temporal resolution in an aberration-corrected transmission electron microscope

Exploring the corresponding relation between structural and physical properties of materials at the atomic scale remains the fundamental problem in science. With the development of the aberration-corrected transmission electron microscopy (AC-TEM) and the ultrafast optical spectroscopy technique, sub-angstrom-scale spatial resolution and femtosecond-scale temporal resolution can be achieved, respectively. However, the attempt to combine both their advantages is still a great challenge. Here, we develop in situ optical spectroscopy with high temporal resolution in AC-TEM by utilizing a self-designed and manufactured TEM specimen holder, which has the capacity of sub-angstrom-scale spatial resolution and femtosecond-scale temporal resolution. The key and unique design of our apparatus is the use of the fiber bundle, which enables the delivery of focused pulse beams into TEM and collection of optical response simultaneously. The generated focused spot has a size less than 2 µm and can be scanned in plane with an area larger than 75 × 75 µm². Most importantly, the positive group-velocity dispersion caused by glass fiber is compensated by a pair of diffraction gratings, thus resulting in the generation of pulse beams with a pulse width of about 300 fs (@ 3 mW) in TEM. The in situ experiment, observing the atomic structure of CdSe/ZnS quantum dots in AC-TEM and obtaining the photoluminescence lifetime (∼4.3 ns) in the meantime, has been realized. Further ultrafast optical spectroscopy with femtosecond-scale temporal resolution could be performed in TEM by utilizing this apparatus.

Tin dioxide-based nanomaterials as anodes for lithium-ion batteries

The development of new electrode materials for lithium-ion batteries (LIBs) has attracted significant attention because commercial anode materials in LIBs, like graphite, may not be able to meet the increasing energy demand of new electronic devices. Tin dioxide (SnO2) is considered as a promising alternative to graphite due to its high specific capacity. However, the large volume changes of SnO2 during the lithiation/delithiation process lead to capacity fading and poor cycling performance. In this review, we have summarized the synthesis of SnO2-based nanomaterials with various structures and chemical compositions, and their electrochemical performance as LIB anodes. This review addresses pure SnO2 nanomaterials, the composites of SnO2 and carbonaceous materials, the composites of SnO2 and transition metal oxides, and other hybrid SnO2-based materials. By providing a discussion on the synthesis methods and electrochemistry of some representative SnO2-based nanomaterials, we aim to demonstrate that electrochemical properties can be significantly improved by modifying chemical composition and morphology. By analyzing and summarizing the recent progress in SnO2 anode materials, we hope to show that there is still a long way to go for SnO2 to become a commercial LIB electrode and more research has to be focused on how to enhance the cycling stability.

In Situ Imaging Polysulfides Electrochemistry of Li-S Batteries in a Hollow Carbon Nanotubule Wet Electrochemical Cell

Understanding polysulfide electrochemistry is critical for mitigation of the polysulfide shuttle effect in Li-S batteries. However, in situ imaging polysulfides evolution in Li-S batteries has not been possible. Herein, we constructed a hollow carbon nanotubule (CNT) wet electrochemical cell that permits real-time imaging of polysulfide evolutions in Li-S batteries in a Cs-corrected environmental transmission electron microscope. Upon discharge, sulfur was electrochemically reduced to long-chain polysulfides, which dissolved into the electrolyte instantly and were stabilized by Py₁₄⁺ cations solvation. Metastable polysulfides prove to be problematic for Li-S batteries, therefore, destabilizing the Py₁₄⁺-solvated polysulfides by adding low polarized solvents into the electrolyte to weaken the interaction between Py₁₄⁺ cation and long-chain polysulfides renders a rapid polysulfides-to-Li₂S transition, thus efficiently mitigating polysulfide formation and improving the performance of Li-S batteries dramatically. Moreover, the CNT wet electrochemical cell proves to be a universal platform for in situ probing electrochemistry of various batteries.

Operando surface science methodology reveals surface effect in charge storage electrodes

Surface and interface play critical roles in energy storage devices, calling for operando characterization techniques to probe the electrified surfaces/interfaces. In this work, surface science methodology, including electron spectroscopy and scanning probe microscopy, has been successfully applied to visualize electrochemical processes at operating electrode surfaces in an Al/graphite model battery. Intercalation of anions together with cations is directly observed in the surface region of a graphite electrode with tens of nanometers thickness, the concentration of which is one order higher than that in bulk. An intercalation pseudocapacitance mechanism and a double specific capacity in the electrode surface region are expected based on the super-dense intercalants and anion/cation co-intercalation, which are in sharp contrast to the battery-like mechanism in the electrode bulk. The distinct electrochemical mechanism at the electrode surface is verified by performance tests of real battery devices, showing that a surface-dominant, nanometer-thick graphite cathode outperforms a bulk-dominant, micrometer-thick graphite cathode. Our findings highlight the important surface effect of working electrodes in charge storage systems.

Diffusion Along Dislocations Mitigates Self-Limiting Na Diffusion in Crystalline Sn

The diffusion of carrier ions in alloying anodes often develops compressive stresses in front of the propagating interface, suppressing the carrier-ion diffusion and limiting their full penetration into alloying anodes during battery cycles. This phenomenon, termed "self-limiting diffusion (SLD)", reduces the rate performance of batteries and hinders the full usage of anode materials. However, SLD is mitigated in some systems where tensile residual stresses develop at the interface, causing them to manifest significantly improved rate performance and energy capacity. Here, a comparative study of LiSi and NaSn systems to elucidate how the differing diffusion kinetics displayed by the two systems can influence SLD behaviors and the rate performance of batteries is performed. Experiments show that the Na diffusion into soft Sn crystals induces tensile stresses near the interface, promoting the nucleation of high-density dislocations. Thus-formed dislocations facilitate Na diffusion at ultrafast rates by providing pathways for dislocation pipe diffusion and alleviate SLD, making crystalline Sn suitable for fast-charging anode material. The outcomes of this study, while filling the knowledge gaps on the reasons for SLD, offer some guidelines for the appropriate choice of potential anode materials with superior rate performance and energy capacity suitable for future applications.

Atomic-Level Manipulations in Oxides and Alloys for Electrocatalysis of Oxygen Evolution and Reduction

As chemical reactions and charge-transfer simultaneously occur on the catalyst surface during electrocatalysis, numerous studies have been carried out to attain an in-depth understanding on the correlation among the surface structure and composition, the electrical transport, and the overall catalytic activity. Compared with other catalysis reactions, a relatively larger activation barrier for oxygen evolution/reduction reactions (OER/ORR), where multiple electron transfers are involved, is noted. Many works over the past decade thus have been focused on the atomic-scale control of the surface structure and the precise identification of surface composition change in catalyst materials to achieve better conversion efficiency. In particular, recent advances in various analytical tools have enabled noteworthy findings of unexpected catalytic features at atomic resolution, providing significant insights toward reducing the activation barriers and subsequently improving the catalytic performance. In addition to summarizing important surface issues, including lattice defects, related to the OER and ORR in this Review, we present the current status and discuss future perspectives of oxide- and alloy-based catalysts in terms of atomic-scale observation and manipulation.

Electrolyte-Phobic Surface for the Next-Generation Nanostructured Battery Electrodes

Nanostructured electrodes are among the most important candidates for high-capacity battery chemistry. However, the high surface area they possess causes serious issues. First, it would decrease the Coulombic efficiencies. Second, they have significant intakes of liquid electrolytes, which reduce the energy density and increase the battery cost. Third, solid-electrolyte interphase growth is accelerated, affecting the cycling stability. Therefore, the interphase chemistry regarding electrolyte contact is crucial, which was rarely studied. Here, we present a completely new strategy of limiting effective surface area by introducing an "electrolyte-phobic surface". Using this method, the electrolyte intake was limited. The initial Coulombic efficiencies were increased up to ∼88%, compared to ∼60% of the control. The electrolyte-phobic layer of Si particles is also compatible with the binder, stabilizing the electrode for long-term cycling. This study advances the understanding of interphase chemistry, and the introduction of the universal concept of electrolyte-phobicity benefits the next-generation battery designs.

Stress-resilient electrode materials for lithium-ion batteries: strategies and mechanisms

Next-generation high-performance lithium-ion batteries (LIBs) with high energy and power density, long cycle life and uncompromising safety standards require new electrode materials beyond conventional intercalation compounds. However, these materials face a tradeoff between the high capacity and stable cycling because more Li stored in the materials also brings instability to the electrode. Stress-resilient electrode materials are the solution to balance this issue, where the decoupling of strong chemomechanical effects on battery cycling is a prerequisite. This review covers the (de)lithiation behaviors of the alloy and conversion-type anodes and their stress mitigation strategies. We highlight the reaction and degradation mechanisms down to the atomic scale revealed by in situ methods. We also discuss the implications of these mechanistic studies and comment on the effectiveness of the electrode structural and chemical designs that could potentially enable the commercialization of the next generation LIBs based on high-capacity anodes.

Unraveling the Reaction Mechanism of FeS2 as a Li-Ion Battery Cathode

Iron pyrite (FeS₂) is a promising lithium-ion battery cathode material because of its low cost and ultrahigh energy density (1671 Wh kg^-1). However, its reaction mechanisms are still controversial. In this work, we find that different from the conventional belief that an intermediate phase Li₂FeS₂ is formed followed by Fe/Li₂S composites at the initial discharge, it undergoes a one-step reaction (FeS₂ → Fe + Li₂S) or a two-step reaction (FeS₂ → FeS + Li₂S → Fe + Li₂S), which depends on the current rate and temperature. In the charge process, it undergoes a two-step reaction: phase transition Fe + Li₂S → FeS at about 1.74 V and generation of elemental sulfur (Li₂S → S, 2.30 V). FeS is a mackinawite phase that is formed on the interface of Li₂S via heteroepitaxial growth. Subsequent cycles involves a combination reaction of FeS and S. The reaction mechanism suggests that FeS₂ suffers from the demerits of both FeS and S, such as a large volume change, voltage hysteresis, and polysulfide dissolution. These findings would help us to understand the intrinsic capacity fading of FeS₂ and provide guidelines to improve its electrochemical performances.

Decoding of Oxygen Network Distortion in a Layered High-Rate Anode by In Situ Investigation of a Single Microelectrode

Sluggish conversion reactions severely impair the rate capability for lithium storage, which is the main disadvantage of the conversion-type anode materials. Here, the microplatform based on a single microelectrode is designed and utilized for the fundamental understanding of the conversion reaction. The kinetic-favorable layered structure of the anode material is on-site synthesized in the microplatform. The in situ characterization reveals that introducing an oxygen network distortion in the layered oxide anode effectively circumvents the severe passivation of the electrode material by lithium oxide, thus leading to highly reversible conversion reactions. As a result, the high-rate capability of the conversion-type anode materials is realized. The on-site synthesis strategy is further applied in the large-scale synthesis of nanomaterials for lithium-ion batteries. As such, oxide nanorods with the layered structure are synthesized by a facile chemical strategy, showing high rate performance (574 mAh g^-1 at 10 A g^-1). This work unveils the beneficial effect of oxygen network distortion in the layered anode for conversion reactions over cycling, thus providing an alternative strategy to enhance the rate capability of conversion-type anodes for lithium storage.

A rationally designed 3D interconnected porous tin dioxide cube with reserved space for volume expansion as an advanced anode of lithium-ion batteries

To work against the volume expansion (∼300%) of SnO₂ during lithiation, here a sub-micro sized, interconnected, and porous SnO₂ cube with rationally designed reserved space (∼375%) is synthesized via an artful topochemistry route (CaSn(OH)₆-CaSnO₃-SnO₂). Owing to its microstructure, this novel material harvests enhanced lithium-storage performance.

Opportunities for Cryogenic Electron Microscopy in Materials Science and Nanoscience

Cryogenic electron microscopy (cryo-EM) was the basis for the 2017 Nobel Prize in Chemistry for its profound impact on the field of structural biology by freezing and stabilizing fragile biomolecules for near atomic-resolution imaging in their native states. Beyond life science, the development of cryo-EM for the physical sciences may offer access to previously inaccessible length scales for materials characterization in systems that would otherwise be too sensitive for high-resolution electron microscopy and spectroscopy. Weakly bonded and reactive materials that typically degrade under electron irradiation and environmental exposure can potentially be stabilized by cryo-EM, opening up exciting opportunities to address many central questions in materials science. New discoveries and fundamental breakthroughs in understanding are likely to follow. In this Perspective, we identify six major areas in materials science that may benefit from the interdisciplinary application of cryo-EM: (1) batteries, (2) soft polymers, (3) metal-organic frameworks, (4) perovskite solar cells, (5) electrocatalysts, and (6) quantum materials. We highlight long-standing questions in each of these areas that cryo-EM can potentially address, which would firmly establish the powerful tool's broad scope and utility beyond biology.

Multi-Step Phase Transitions of Mn3 O4 During Galvanostatic Lithiation: An In Situ Transmission Electron Microscopic Investigation

For study of electrochemical reaction mechanisms at nanoscale, in situ electrochemical transmission electron microscopy (EC-TEM) exceeds many other methods due to its high temporal and spatial resolution. However, the limited amount of active materials used in previous in situ TEM studies prevents the model EC cells to operate in the constant-current (galvanostatic) charge/discharge mode that is required for accurate control of electrochemical processes. Herein, a new in situ EC-TEM technique is developed to investigate multi-step phase transitions of Mn₃ O₄ electrodes under the galvanostatic charge/discharge mode and constant-voltage discharge mode. In galvanostatic mode, the lithiation of Mn₃ O₄ undergoes multi-step phase transitions following a reaction pathway of Mn₃ O₄ + Li⁺ → LiMn₃ O₄ + Li⁺ → MnO + Li₂ O → Mn + Li₂ O. It is also found that lithium ions prefer to enter Mn₃ O₄ along the {101} direction to form LiMn₃ O₄ with the help of transitional boundary phase of Li_x Mn₃ O₄ . These results are in sharp contrast to that obtained under a constant-voltage discharge mode, where only a single-step lithiation process of Mn₃ O₄ + Li⁺ → Mn + Li₂ O is observed.

Conformal Shell Amorphization of Nanoporous Ag-Bi for Efficient Formate Generation

Simultaneous attainments of high conductivity and superior catalysis are major challenges for amorphous electrocatalysts in carbon dioxide electroreduction at high overpotential. In this study, one protocol is first demonstrated to drive the shell amorphization of nanoporous Ag-Bi (a-NPSB) catalyst with the spatially interconnected ligament during the initial stage of CO₂ER. This newborn a-NPSB bestows the outstanding catalysis, evidenced by a Faradaic efficiency of 88.4% for formate production at -1.15 V vs RHE, specific current density of 21.2 mA cm^-2, and mass specific current density of 321 mA mg^-1. The unique catalysis is considered as the collective contribution of the conductive ligament internally and amorphous Bi₂O₃ shell with about 3.2 nm thickness externally. Simultaneous obtaining of the conductivity of inner metals and catalytic activity of the amorphous shell will pave a new avenue for designing a robust electrode during electrochemical reaction.

Unveiling the Advances of Nanostructure Design for Alloy-Type Potassium-Ion Battery Anodes via In Situ TEM

Nanostructure design and in situ transmission electron microscopy (TEM) are combined to demonstrate Sb-based nanofibers composed of bunched yolk-shell building units as a significantly improved anode for potassium-ion batteries (PIBs). Particularly, a metal-organic frameworks (MOFs)-engaged electrospinning strategy coupled to a confined ion-exchange followed by a subsequent thermal reduction is proposed to fabricate yolk-shell Sb@C nanoboxes embedded in carbon nanofibers (Sb@CNFs). In situ TEM analysis reveals that the inner Sb nanoparticles undergo a significant volume expansion/contraction during the alloying/dealloying processes, while the void space can effectively relieve the overall volume change, and the plastic carbon shell maintains the structural integrity of electrode material. This work provides an important reference for the application of advanced characterization techniques to guide the optimization of electrode material design.

Dynamic imaging of lithium in solid-state batteries by operando electron energy-loss spectroscopy with sparse coding

Lithium-ion transport in cathodes, anodes, solid electrolytes, and through their interfaces plays a crucial role in the electrochemical performance of solid-state lithium-ion batteries. Direct visualization of the lithium-ion dynamics at the nanoscale provides valuable insight for understanding the fundamental ion behaviour in batteries. Here, we report the dynamic changes of lithium-ion movement in a solid-state battery under charge and discharge reactions by time-resolved operando electron energy-loss spectroscopy with scanning transmission electron microscopy. Applying image denoising and super-resolution via sparse coding drastically improves the temporal and spatial resolution of lithium imaging. Dynamic observation reveals that the lithium ions in the lithium cobaltite cathode are complicatedly extracted with diffusion through the lithium cobaltite domain boundaries during charging. Even in the open-circuit state, they move inside the cathode. Operando electron energy-loss spectroscopy with sparse coding is a promising combination to visualize the ion dynamics and clarify the fundamentals of solid-state electrochemistry.

Understanding lithium ion dynamics holds the key to unlocking better battery materials and devices. Here, by combining electron energy-loss spectroscopy and machine learning, the authors reveal how lithium is extracted from LiCoO₂ cathode used in a solid-state battery.

Spontaneous and reversible hollowing of alloy anode nanocrystals for stable battery cycling

High-capacity alloy anode materials for Li-ion batteries have long been held back by limited cyclability caused by the large volume changes during lithium insertion and removal. Hollow and yolk-shell nanostructures have been used to increase the cycling stability by providing an inner void space to accommodate volume changes and a mechanically and dimensionally stable outer surface. These materials, however, require complex synthesis procedures. Here, using in situ transmission electron microscopy, we show that sufficiently small antimony nanocrystals spontaneously form uniform voids on the removal of lithium, which are then reversibly filled and vacated during cycling. This behaviour is found to arise from a resilient native oxide layer that allows for an initial expansion during lithiation but mechanically prevents shrinkage as antimony forms voids during delithiation. We developed a chemomechanical model that explains these observations, and we demonstrate that this behaviour is size dependent. Thus, antimony naturally evolves to form optimal nanostructures for alloy anodes, as we show through electrochemical experiments in a half-cell configuration in which 15-nm antimony nanocrystals have a consistently higher Coulombic efficiency than larger nanoparticles.

Analyzing Energy Materials by Cryogenic Electron Microscopy

Safe and high-energy-density rechargeable batteries are increasingly indispensable in the pursuit of a wireless and fossil-free society. Advancements in present battery technologies and the investigation of next-generation batteries highly depend on the ever-deepening fundamental understanding and the rational designs of working electrodes, electrolytes, and interfaces. However, accurately analyzing energy materials and interfaces is severely hindered by their intrinsic limitations of air and electron-beam sensitivity, which restrains the research of energy materials in a low-efficiency trial-and-error paradigm. The emergence of cryogenic electron microscopy (cryo-EM) has enabled the nondestructive characterization of air- and electron-beam sensitive energy materials in the microscale and nanoscale, and even at atomic resolutions, affording closer insights into the primary chemistry and physics of working batteries. Herein, the development of cryo-EM and the applications in detecting energy materials are reviewed and analyzed from its overwhelming advantages in disclosing the underlying mystery of energy materials. Critical sample preparation methods as the precondition for cryo-EM are compared, which strongly affect the characterization accuracy. Furthermore, new developments in the analysis of energy materials, especially bulk electrodes and interfaces in lithium metal batteries, are presented according to different functions of cryo-EM. Finally, future directions of cryo-EM for analyzing energy materials are prospected.

Visualization of Electrochemical Cycling-Induced Dimension Change in LiMn2O4 Nanoparticles by High-Speed Atomic Force Microscopy

Exploring dynamic dimension change and lithium-ion diffusion kinetics of active nanoparticles is important to further improve the qualities of lithium-ion batteries (LIBs), such as the cycle life and charge rate. For advancing such research, an imaging technique that is capable of operating in an electrochemical environment with high spatial and temporal resolutions is really needed. In this work, we successfully developed electrochemical high-speed atomic force microscopy (EC-HS-AFM), which enabled nanoscale imaging at the rate of ∼1 frame/s during electrochemical cycling. The dimensional evolutions of LiMn₂O₄ single nanoparticles accompanying an insertion/extraction reaction of lithium ions were visualized. The surface area-potential hysteresis loops of the single nanoparticles at different sweep rates were quantitatively extracted from the successive HS-AFM images or video. The first-order derivative of the hysteresis loop was interestingly similar to the cyclic voltammetry (CV). Moreover, the EC-HS-AFM experiments confirmed that the utilization of the nanoparticles in the cathode can indeed improve the rate performance of the LIBs. These results demonstrated that EC-HS-AFM would be a promising tool to study dimensional evolutions and lithium-ion diffusion kinetics at a nanoscale.

Thermodynamic analysis and kinetic optimization of high-energy batteries based on multi-electron reactions

Multi-electron reaction can be regarded as an effective way of building high-energy systems (>500 W h kg⁻¹). However, some confusions hinder the development of multi-electron mechanisms, such as clear concept, complex reaction, material design and electrolyte optimization and full-cell fabrication. Therefore, this review discusses the basic theories and application bottlenecks of multi-electron mechanisms from the view of thermodynamic and dynamic principles. In future, high-energy batteries, metal anodes and multi-electron cathodes are promising electrode materials with high theoretical capacity and high output voltage. While the primary issue for the multi-electron transfer process is sluggish kinetics, which may be caused by multiple ionic migration, large ionic radius, high reaction energy barrier, low electron conductivity, poor structural stability, etc., it is urgent that feasible and versatile modification methods are summarized and new inspiration proposed in order to break through kinetic constraints. Finally, the remaining challenges and future research directions are revealed in detail, involving the search for high-energy systems, compatibility of full cells, cost control, etc.

Electron microscopy using ionic liquids for life and materials sciences

An ionic liquid (IL) is a salt consisting of only cations and anions, which exists in the liquid state at room temperature. Interestingly ILs combine various favorable physicochemical properties, such as negligible vapor pressure, flame resistance, relatively high ionic conductivity, wide electrochemical window, etc. To take advantage of two specific features of ILs, viz. their nonvolatile and antistatic nature, in 2006, Kuwabata, Torimoto et al. reported a milestone study led to current IL-based electron microscopy techniques. Thereafter, several IL-based electron microscopy techniques have been proposed for life science and materials science applications, e.g. pretreatment of hydrous and/or non-electron conductive specimens and in situ/operando observation of chemical reactions occurring in ILs. In this review, the fundamental approaches for making full use of these techniques and their impact on science and technology are introduced.

Intercalating Sn/Fe Nanoparticles in Compact Carbon Monolith for Enhanced Lithium Ion Storage

Given its high-capacity of multielectron (de-)lithiation, SnO2 is deemed as a competitive anode substance to tackle energy density restrictions of low-theoretical-capacity traditional graphite. However, its pragmatic adhibition seriously encounters poor initial coulombic efficiency from irreversible Li2O formation and drastic volume change during repeated charge/discharge. Here, an applicable gel pyrolysis methodology establishes a SnO2/Fe2O3 intercalated carbon monolith as superior anode materials for Li ion batteries to effectively surmount problems of SnO2. Its bulk-like, micron-sized, compact, and non-porous structures with low area surfaces (14.2 m2 g−1) obviously increase the tap density without compromising the transport kinetics, distinct from myriad hierarchically holey metal/carbon materials recorded till date. During the long-term Li+ insertion/extraction, the carbon matrix not only functions as a stress management framework to alleviate the stress intensification on surface layers, enabling the electrode to retain its morphological/mechanic integrity and yielding a steady solid electrolyte interphase film, but also imparts very robust connection to stop SnO2 from coarsening/losing electric contact, facilitating fast electrolyte infiltration and ion/electron transfer. Besides, the closely contacted and evenly distributed Fe2O3/SnO2 nanoparticles supply additional charge-transfer driving force, thanks to a built-in electric field. Benefiting from such virtues, the embedment of binary metal oxides in the dense carbons enhances initial Coulombic efficiency up to 67.3%, with an elevated reversible capacity of 726 mAh/g at 0.2 A/g, a high capacity retention of 84% after 100 cycles, a boosted rate capability between 0.2 and 3.2 A g−1, and a stable cycle life of 466 mAh/g over 200 cycles at 1 A g−1. Our scenario based upon this unique binary metal-in-carbon sandwich compact construction to achieve the stress regulation and the so-called synergistic effect between metals or metal oxides and carbons is economically effective and tractable enough to scale up the preparation and can be rifely employed to other oxide anodes for ameliorating their electrochemical properties.

SnO2 Quantum Dots@Graphene Framework as a High-Performance Flexible Anode Electrode for Lithium-Ion Batteries

Three-dimensional (3D) layered tin oxide quantum dots/graphene framework (SnO₂ QDs@GF) were designed through anchoring SnO₂ QD on the graphene surface under the hydrothermal reaction. SnO₂ QDs@GF have a 3D skeleton with a large number of mesopores and ultrasmall SnO₂ QDs with a large surface area. The unique design of this structure improves the specific area and promotes ion transport. The mechanically strong SnO₂ QDs@GF can directly be used as the anode of lithium-ion batteries (LIBs); it displays a high reversible capacity (1300 mA h g^-1 at 100 mA g^-1), excellent rate performance (642 mA h g^-1 at 2000 mA g^-1), and superior cyclic stability (when the current density is 10 A g^-1, the capacity loss is less than 2% after 5000 cycles). This novel synthetic method can further be expanded for the production of other quantum dots/graphene composites with a 3D structure as high-performance electrodes for LIBs.

Direct Visualization of Lithium Polysulfides and Their Suppression in Liquid Electrolyte

Understanding of lithium polysulfide (Li-PS) formation and the shuttle phenomenon is essential for practical application of the lithium/sulfur (Li/S) cell, which has superior theoretical specific energy (2600 Wh/kg). However, it suffers from the lack of direct observation on behaviors of soluble Li-PS in liquid electrolytes. Using in situ graphene liquid cell electron microscopy, we have visualized formation and diffusion of Li-PS simultaneous with morphological and phase evolutions of sulfur nanoparticles during lithiation. We found that the morphological changes and Li-PS diffusion are retarded by ionic liquid (IL) addition into electrolyte. Chronoamperometric shuttle current measurement confirms that IL addition lowers the experimental diffusion coefficient of Li-PS by 2 orders of magnitude relative to that in IL-free electrolyte and thus suppresses the Li-PS shuttle current, which accounts for better cyclability and Coulombic efficiency of the Li/S cell. This study provides significant insights into electrolyte design to inhibit the polysulfide shuttle phenomenon.

Surface modification of tin oxide through reduced graphene oxide as a highly efficient cathode material for magnesium-ion batteries

Among post-lithium ion technologies, magnesium-ion batteries (MIBs) are receiving great concern in recent years. However, MIBs are mainly restrained by the lack of cathode materials, which may accommodate the fast diffusion kinetics of Mg²⁺ ions. To overcome this problem, herein we attempt to synthesize a reduced graphene oxide (rGO) encapsulated tin oxide (SnO₂) nanoparticles composites through an electrostatic-interaction-induced-self-assembly approach at low temperature. The surface modification of SnO₂ via carbonaceous coating enhanced the electrical conductivity of final composites. The SnO₂-rGO composites with different weight ratios of rGO and SnO₂ are employed as cathode material in magnesium-ion batteries. Experimental results show that MIB exhibits a maximum specific capacity of 222 mAhg^-1 at the current density of 20 mAg^-1 with a good cycle life (capacity retention of 90%). Unlike Li-ion batteries, no SnO₂ nanoparticles expansion is observed during electrochemical cycling in all-phenyl-complex (APC) magnesium electrolytes, which ultimately improves the capacity retention. Furthermore, ex-situ x-ray diffraction and scanning electron microscopy (SEM) studies are used to understand the magnesiation/de-magnesiation mechanisms. At the end, SnO₂-rGO composites are tested for Mg²⁺/Li⁺ hybrid ion batteries and results reveal a specific capacity of 350 mAhg^-1 at the current density of 20 mAg^-1. However, hybrid ion battery exhibited sharp decay in capacity owing to volume expansion of SnO₂ based cathodes. This work will provide a new insight for synthesis of electrode materials for energy storage devices.

Direct Visualization of Electric-Field-Induced Structural Dynamics in Monolayer Transition Metal Dichalcogenides

Layered transition metal dichalcogenides offer many attractive features for next-generation low-dimensional device geometries. Due to the practical and fabrication challenges related to in situ methods, the atomistic dynamics that give rise to realizable macroscopic device properties are often unclear. In this study, in situ transmission electron microscopy techniques are utilized in order to understand the structural dynamics at play, especially at interfaces and defects, in the prototypical film of monolayer MoS₂ under electrical bias. Through our sample fabrication process, we clearly identify the presence of mass transport in the presence of a lateral electric field. In particular, we observe that the voids present at grain boundaries combine to induce structural deformation. The electric field mediates a net vacancy flux from the grain boundary interior to the exposed surface edge sites that leaves molybdenum clusters in its wake. Following the initial biasing cycles, however, the mass flow is largely diminished and the resultant structure remains stable over repeated biasing. We believe insights from this work can help explain observations of nonuniform heating and preferential oxidation at grain boundary sites in these materials.

Electrode Degradation in Lithium-Ion Batteries

Although Li-ion batteries have emerged as the battery of choice for electric vehicles and large-scale smart grids, significant research efforts are devoted to identifying materials that offer higher energy density, longer cycle life, lower cost, and/or improved safety compared to those of conventional Li-ion batteries based on intercalation electrodes. By moving beyond intercalation chemistry, gravimetric capacities that are 2-5 times higher than that of conventional intercalation materials (e.g., LiCoO₂ and graphite) can be achieved. The transition to higher-capacity electrode materials in commercial applications is complicated by several factors. This Review highlights the developments of electrode materials and characterization tools for rechargeable lithium-ion batteries, with a focus on the structural and electrochemical degradation mechanisms that plague these systems.

An Open-Cell Environmental Transmission Electron Microscopy Technique for In Situ Characterization of Samples in Aqueous Liquid Solutions

The desire to image specimens in liquids has led to the development of open-cell and closed-cell techniques in transmission electron microscopy (TEM). The closed-cell approach is currently more common in TEM and has yielded new insights into a number of biological and materials processes in liquid environments. The open-cell approach, which requires an environmental TEM (ETEM), is technically challenging but may be advantageous in certain circumstances due to fewer restrictions on specimen and detector geometry. Here, we demonstrate a novel approach to open-cell liquid TEM, in which we use salt particles to facilitate the in situ formation of droplets of aqueous solution that envelope specimen particles coloaded with the salt. This is achieved by controlling sample temperature between 1 and 10°C and introducing water vapor to the ETEM chamber above the critical pressure for the formation of liquid water on the salt particles. Our use of in situ hydration enables specimens to be loaded into a microscope in a dry state using standard 3 mm TEM grids, allowing specimens to be prepared using trivial sample preparation techniques. Our future aim will be to combine this technique with an in situ light source to study photocorrosion in aqueous environments.

Bulk-Like SnO2-Fe2O3@Carbon Composite as a High-Performance Anode for Lithium Ion Batteries

Boosted power handling and the reduced charging duration of Li ion cells critically rests with ionic/electronic mobility. Ion mobility in electrochemically relevant grains normally stands for an essential restriction of the velocity at which the energy of a cell can be stored/released. To offset sluggish solid-state ionic transport and achieve a superior power/energy density rating, electroactive grains often need exquisite nanoscaling, harming crucial virtues on volumetric packing density, tractability, sustainability, durability, and cost. Unlike elaborate nanostructuring, here we describe that a SnO₂-Fe₂O₃@carbon composite—which adopts a metal oxide particles-intercalated bulk-like configuration—can insert many Li⁺ ions at elevated speeds, despite its micro-dimensionality. Analysis of charge transport kinetics in this tailor-made architecture unveils both much improved ion travel through compact monolithic substances and the greatly enhanced ion access to surfaces of SnO₂/Fe₂O₃ grains. According to the well-studied battery degradation mechanism, it is that both the effective stress management and internal electric field in our as-prepared sample that result in recommendable capacity, rate behavior, and cyclic lifespan (exhibiting a high reversible capacity of 927 mAh g⁻¹ at 0.2 A g⁻¹ with a capacity retention of 95.1% after 100 cycles and an ultra-stable capacity of 429 mAh g⁻¹ even over 1800 cycles at 3 A g⁻¹). Unique materials and working rationale which ensure the swift (de)lithiation of such micrometer-dimensional monoliths may open a door for various high-power/density usages.

General Nanomolding of Ordered Phases

Large-scale, controlled fabrication of ordered phases is challenging at the nanoscale, yet highly demanded as their well-ordered structure and chemistry is the key for advanced functionality. Here, we demonstrate a general nanomolding process of ordered phases based on atomic diffusion. Resulting nanowires are single crystals and maintain their composition and structure throughout their length, which we explain by a self-ordering process originating from their narrow Gibbs free energy. The versatility, control, and precision of this thermomechanical nanomolding method of ordered phases provides new insights into single crystal growth and suggest itself as a technology to enable wide spread usage for nanoscale and quantum devices.

Free-Standing SnO2@rGO Anode via the Anti-solvent-assisted Precipitation for Superior Lithium Storage Performance

Metal oxides have been attractive as high-capacity anode materials for lithium-ion batteries. However, oxide anodes encounter drastic volumetric changes during lithium ion storage through the conversion reaction and alloying/dealloying processes, leading to rapid capacity decay and poor cycling stability. Here, we report a free-standing SnO₂@reduced graphene oxide (SnO₂@rGO) composite anode, in which SnO₂ nanoparticles are tightly wrapped within wrinkled rGO sheets. The SnO₂@rGO sheet is assembled in high porosity via an anti-solvent-assisted precipitation of dispersed SnO₂ nanoparticles and graphene oxide sheets in the distilled water, followed by the filtration and post-annealing processes. Significantly enhanced lithium storage performance has been obtained of the SnO₂@rGO anode compared with the bare SnO₂ anode material. A high charge capacity above 700 mAh g^-1 can be achieved with a satisfying 95.6% retention after 50 cycles at a current density of 500 mA g^-1, superior to reserved 126 mAh g^-1 and a much lower 16.8% retention of the bare SnO₂ anode. XRD pattern and HRTEM images of the cycled SnO₂@rGO anode material verify the expected oxidation of Sn to SnO₂ at the fully-charged state in the 50th cycle. In addition, FESEM and TEM images reveal the well-preserved free-standing structure after cycling, which accounts for high reversible capacity and excellent cycling stability of such a SnO₂@rGO anode. This work provides a promising SnO₂-based anode for high-capacity lithium-ion batteries, together with an effective fabrication adoptable to prepare different free-standing composite materials for device applications.

Liquid cell transmission electron microscopy and its applications

Transmission electron microscopy (TEM) has long been an essential tool for understanding the structure of materials. Over the past couple of decades, this venerable technique has undergone a number of revolutions, such as the development of aberration correction for atomic level imaging, the realization of cryogenic TEM for imaging biological specimens, and new instrumentation permitting the observation of dynamic systems in situ. Research in the latter has rapidly accelerated in recent years, based on a silicon-chip architecture that permits a versatile array of experiments to be performed under the high vacuum of the TEM. Of particular interest is using these silicon chips to enclose fluids safely inside the TEM, allowing us to observe liquid dynamics at the nanoscale. In situ imaging of liquid phase reactions under TEM can greatly enhance our understanding of fundamental processes in fields from electrochemistry to cell biology. Here, we review how in situ TEM experiments of liquids can be performed, with a particular focus on microchip-encapsulated liquid cell TEM. We will cover the basics of the technique, and its strengths and weaknesses with respect to related in situ TEM methods for characterizing liquid systems. We will show how this technique has provided unique insights into nanomaterial synthesis and manipulation, battery science and biological cells. A discussion on the main challenges of the technique, and potential means to mitigate and overcome them, will also be presented.

Atomistic and dynamic structural characterizations in low-dimensional materials: recent applications of in situ transmission electron microscopy

In situ transmission electron microscopy has achieved remarkable advances for atomic-scale dynamic analysis in low-dimensional materials and become an indispensable tool in view of linking a material's microstructure to its properties and performance. Here, accompanied with some cutting-edge researches worldwide, we briefly review our recent progress in dynamic atomistic characterization of low-dimensional materials under external mechanical stress, thermal excitations and electrical field. The electron beam irradiation effects in metals and metal oxides are also discussed. We conclude by discussing the likely future developments in this area.