Lithiation-induced embrittlement of multiwalled carbon nanotubes

Growth and Performance of High-Quality SWCNT Forests on Inconel Foils as Lithium-Ion Battery Anodes

Large-scale production of vertically aligned single-walled carbon nanotubes (VA-SWCNTs) on metal foils promises to enable technological advancements in many fields, from functional composites to energy storage to thermal interfaces. In this work, we demonstrate growth of high-quality (G/D > 6, average diameters ∼ 2-3 nm, densities > 10¹² cm^-2) VA-SWCNTs on Inconel metal for use as a lithium-ion battery (LIB) anode. Scale-up of SWCNT growth on Inconel 625 to 100 cm² exhibits nearly invariant CNT structural properties, even when synthesis is performed near atmospheric pressure, and this robustness is attributed to a growth kinetic regime dominated by the carbon precursor diffusion in the bulk gas mixture. SWCNT forests produced on large-area metal substrates at close to atmospheric pressure possess a combination of structural features that are among the best demonstrated so far in the literature for growth on metal foils. Leveraging these achievements for energy applications, we demonstrate a VA-SWCNT LIB anode with capacity >1200 mAh/g at 1.0C and stable cycling beyond 300 cycles. This robust synthesis of high-quality VA-SWCNTs on metal foils presents a promising route toward mass production of high-performance CNT devices for a broad range of applications.

An Encapsulation-Based Sodium Storage via Zn-Single-Atom Implanted Carbon Nanotubes

The properties of high theoretical capacity, low cost, and large potential of metallic sodium (Na) has strongly promoted the development of rechargeable sodium-based batteries. However, the issues of infinite volume variation, unstable solid electrolyte interphase (SEI), and dendritic sodium causes a rapid decline in performance and notorious safety hazards. Herein, a highly reversible encapsulation-based sodium storage by designing a functional hollow carbon nanotube with Zn single atom sites embedded in the carbon shell (Zn_SA -HCNT) is achieved. The appropriate tube space can encapsulate bulk sodium inside; the inner enriched Zn_SA sites provide abundant sodiophilic sites, which can evidently reduce the nucleation barrier of Na deposition. Moreover, the carbon shell derived from ZIF-8 provides geometric constraints and excellent ion/electron transport channels for the rapid transfer of Na⁺ due to its pore-rich shell, which can be revealed by in situ transmission electron microscopy (TEM). As expected, Na@Zn_SA -HCNT anodes present steady long-term performance in symmetrical battery (>900 h at 10 mA cm^-2 ). Moreover, superior electrochemical performance of Na@Zn_SA -HCNT||PB full cells can be delivered. This work develops a new strategy based on carbon nanotube encapsulation of metallic sodium, which improves the safety and cycling performance of sodium metal anode.

Mechanistic Probing of Encapsulation and Confined Growth of Lithium Crystals in Carbonaceous Nanotubes

Encapsulation of lithium in the confined spaces within individual nanocapsules is intriguing and highly desirable for developing high-performance Li metal anodes. This work aims for a mechanistic understanding of Li encapsulation and its confined growth kinetics inside 1D enclosed spaces. To achieve this, amorphous carbon nanotubes are employed as a model host using in situ transmission electron microscopy. The carbon shells have dual roles, providing geometric/mechanical constraints and electron/ion transport channels, which profoundly alter the Li growth patterns. Li growth/dissolution takes place via atom addition/removal at the free surfaces through Li⁺ diffusion along the shells in the electric field direction, resulting in the formation of unusual Li structures, such as poly-crystalline nanowires and free-standing 2D ultrathin (1-2 nm) Li membranes. Such confined front-growth processes are dominated by Li {110} or {200} growing faces, distinct from the root growth of single-crystal Li dendrites outside the nanotubes. Controlled experiments show that high lithiophilicity/permeability, enabled by sufficient nitrogen/oxygen doping or pre-lithiation, is critical for the stable encapsulation of lithium inside carbonaceous nanocapsules. First-principles-based calculations reveal that N/O doping can reduce the diffusion barrier for Li⁺ penetration, and facilitate Li filling driven by energy minimization associated with the formation of low-energy Li/C interfaces.

Regulating Interfacial Lithium Ion by Artificial Protective Overlayers for High-Performance Lithium Metal Anodes

The main limitation of lithium (Li) metal anodes lies in their severe dendrite growth due to nonuniform Li ion flux and sluggish Li ion transportation at the anode surface. Fabricating artificial protective overlayer with tunable surficial properties on Li metal is a precise and effective strategy to relieve this problem. In this Concept article, we focus on the basic principles of regulating interfacial Li ion through artificial protective overlayers and summarize the material preparation as well as structural design of these overlayers. The remaining challenges and promising directions of artificial protective overlayers are then highlighted to provide clues for the practical application of Li metal anodes.

Lithium Deposition-Induced Fracture of Carbon Nanotubes and Its Implication to Solid-State Batteries

The increasing demand for safe and dense energy storage has shifted research focus from liquid electrolyte-based Li-ion batteries toward solid-state batteries (SSBs). However, the application of SSBs is impeded by uncontrollable Li dendrite growth and short circuiting, the mechanism of which remains elusive. Herein, we conceptualize a scheme to visualize Li deposition in the confined space inside carbon nanotubes (CNTs) to mimic Li deposition dynamics inside solid electrolyte (SE) cracks, where the high-strength CNT walls mimic the mechanically strong SEs. We observed that the deposited Li propagates as a creeping solid in the CNTs, presenting an effective pathway for stress relaxation. When the stress-relaxation pathway is blocked, the Li deposition-induced stress reaches the gigapascal level and causes CNT fracture. Mechanics analysis suggests that interfacial lithiophilicity critically governs Li deposition dynamics and stress relaxation. Our study offers critical strategies for suppressing Li dendritic growth and constructing high-energy-density, electrochemically and mechanically robust SSBs.

Carbon Anode Materials for Rechargeable Alkali Metal Ion Batteries and in-situ Characterization Techniques

Lithium-ion batteries (LIBs), used for energy supply and storage equipment, have been widely applied in consumer electronics, electric vehicles, and energy storage systems. However, the urgent demand for high energy density batteries and the shortage of lithium resources is driving scientists to develop high-performance materials and find alternatives. Low-volume expansion carbon material is the ideal choice of anode material. However, the low specific capacity has gradually become the shortcoming for the development of LIBs and thus developing new carbon material with high specific capacity is urgently needed. In addition, developing alternatives of LIBs, such as sodium ion batteries and potassium-ion batteries, also puts forward demands for new types of carbon materials. As is well-known, the design of high-performance electrodes requires a deep understanding on the working mechanism and the structural evolution of active materials. On this issue, ex-situ techniques have been widely applied to investigate the electrode materials under special working conditions, and provide a lot of information. Unfortunately, these observed phenomena are difficult to reflect the reaction under real working conditions and some important short-lived intermediate products cannot be captured, leading to an incomplete understanding of the working mechanism. In-situ techniques can observe the changes of active materials in operando during the charge/discharge processes, providing the concrete process of solid electrolyte formation, ions intercalation mechanism, structural evolutions, etc. Herein, this review aims to provide an overview on the characters of carbon materials in alkali ion batteries and the role of in-situ techniques in developing carbon materials.

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.

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.

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.

In Situ Transmission Electron Microscopy for Energy Materials and Devices

Energy devices such as rechargeable batteries, fuel cells, and solar cells are central to powering a renewable, mobile, and electrified future. To advance these devices requires a fundamental understanding of the complex chemical reactions, material transformations, and charge flow that are associated with energy conversion processes. Analytical in situ transmission electron microscopy (TEM) offers a powerful tool for directly visualizing these complex processes at the atomic scale in real time and in operando. Recent advancements in energy materials and devices that have been enabled by in situ TEM are reviewed. First, the evolutionary development of TEM nanocells from the open-cell configuration to the closed-cell, and finally the full-cell, is reviewed. Next, in situ TEM studies of rechargeable ion batteries in a practical operation environment are explored, followed by applications of in situ TEM for direct observation of electrocatalyst formation, evolution, and degradation in proton-exchange membrane fuel cells, and fundamental investigations of new energy materials such as perovskites for solar cells. Finally, recent advances in the use of environmental TEM and cryogenic electron microscopy in probing clean-energy materials are presented and emerging opportunities and challenges in in situ TEM research of energy materials and devices are discussed.

Review of Recent Development of In Situ/Operando Characterization Techniques for Lithium Battery Research

The increasing demands of energy storage require the significant improvement of current Li-ion battery electrode materials and the development of advanced electrode materials. Thus, it is necessary to gain an in-depth understanding of the reaction processes, degradation mechanism, and thermal decomposition mechanisms under realistic operation conditions. This understanding can be obtained by in situ/operando characterization techniques, which provide information on the structure evolution, redox mechanism, solid-electrolyte interphase (SEI) formation, side reactions, and Li-ion transport properties under operating conditions. Here, the recent developments in the in situ/operando techniques employed for the investigation of the structural stability, dynamic properties, chemical environment changes, and morphological evolution are described and summarized. The experimental approaches reviewed here include X-ray, electron, neutron, optical, and scanning probes. The experimental methods and operating principles, especially the in situ cell designs, are described in detail. Representative studies of the in situ/operando techniques are summarized, and finally the major current challenges and future opportunities are discussed. Several important battery challenges are likely to benefit from these in situ/operando techniques, including the inhomogeneous reactions of high-energy-density cathodes, the development of safe and reversible Li metal plating, and the development of stable SEI.

Suppressing Li Metal Dendrites Through a Solid Li-Ion Backup Layer

The growing demand for sustainable and off-grid energy storage is reviving the attempts to use Li metal as the anode in the next generation of batteries. However, the use of Li anodes is hampered due to the growth of Li dendrites upon charging and discharging, which compromises the life and safety of the battery. Here, it is shown that lithiated multiwall carbon nanotubes (Li-MWCNTs) act as a controlled Li diffusion interface that suppresses the growth of Li dendrites by regulating the Li⁺ ion flux during charge/discharge cycling at current densities between 2 and 4 mA cm^-2 . A full Li-S cell is fabricated to showcase the versatility of the protected Li anode with the Li-MWCNT interface, where the full cells could support pulse discharges at high currents and over 450 cycles at different rates with coulombic efficiencies close to 99.9%. This work indicates that carbon materials in lithiated forms can be an effective and simple approach to the stabilization of Li metal anodes.

Kinked silicon nanowires-enabled interweaving electrode configuration for lithium-ion batteries

A tri-dimensional interweaving kinked silicon nanowires (k-SiNWs) assembly, with a Ni current collector co-integrated, is evaluated as electrode configuration for lithium ion batteries. The large-scale fabrication of k-SiNWs is based on a procedure for continuous metal assisted chemical etching of Si, supported by a chemical peeling step that enables the reuse of the Si substrate. The kinks are triggered by a simple, repetitive etch-quench sequence in a HF and H₂O₂-based etchant. We find that the inter-locking frameworks of k-SiNWs and multi-walled carbon nanotubes exhibit beneficial mechanical properties with a foam-like behavior amplified by the kinks and a suitable porosity for a minimal electrode deformation upon Li insertion. In addition, ionic liquid electrolyte systems associated with the integrated Ni current collector repress the detrimental effects related to the Si-Li alloying reaction, enabling high cycling stability with 80% capacity retention (1695 mAh/g_Si) after 100 cycles. Areal capacities of 2.42 mAh/cm² (1276 mAh/g_electrode) can be achieved at the maximum evaluated thickness (corresponding to 1.3 mg_Si/cm²). This work emphasizes the versatility of the metal assisted chemical etching for the synthesis of advanced Si nanostructures for high performance lithium ion battery electrodes.

Liquid Cell Transmission Electron Microscopy

Liquid cell transmission electron microscopy (TEM) has attracted significant interest in recent years. With nanofabricated liquid cells, it has been possible to image through liquids using TEM with subnanometer resolution, and many previously unseen materials dynamics have been revealed. Liquid cell TEM has been applied to many areas of research, ranging from chemistry to physics, materials science, and biology. So far, topics of study include nanoparticle growth and assembly, electrochemical deposition and lithiation for batteries, tracking and manipulation of nanoparticles, catalysis, and imaging of biological materials. In this article, we first review the development of liquid cell TEM and then highlight progress in various areas of research. In the study of nanoparticle growth, the electron beam can serve both as the illumination source for imaging and as the input energy for reactions. However, many other research topics require the control of electron beam effects to minimize electron beam damage. We discuss efforts to understand electron beam-liquid matter interactions. Finally, we provide a perspective on future challenges and opportunities in liquid cell TEM.

Two-Fold Anisotropy Governs Morphological Evolution and Stress Generation in Sodiated Black Phosphorus for Sodium Ion Batteries

Phosphorus represents a promising anode material for sodium ion batteries owing to its extremely high theoretical capacity. Recent in situ transmission electron microscopy studies evidenced anisotropic swelling in sodiated black phosphorus, which may find an origin from the two intrinsic anisotropic properties inherent to the layered structure of black phosphorus: sodium diffusional directionality and insertion strain anisotropy. To understand the morphological evolution and stress generation in sodiated black phosphorus, we develop a chemo-mechanical model by incorporating the intrinsic anisotropic properties into the large elasto-plastic deformation. Our modeling results reveal that the apparent morphological evolution in sodiated black phosphorus is critically controlled by the coupled effect of the two intrinsic anisotropic properties. In particular, sodium diffusional directionality generates sharp interphases along the [010] and [001] directions, which constrain anisotropic development of the insertion strain. The coupled effect renders distinctive stress-generation and fracture mechanisms when sodiation starts from different crystal facets. In addition to providing a powerful modeling framework for sodiation and lithiation of layered structures, our findings shed significant light on the sodiation-induced chemo-mechanical degradation of black phosphorus as a promising anode for the next-generation sodium ion batteries.

In situ analyses for ion storage materials

Development of high performance electrode materials for energy storage is one of the most important issues for our future society. However, a lack of clear analytical views limits critical understanding of electrode materials. This review covers useful analytical work using X-ray diffraction, X-ray absorption spectroscopy, microscopy and neutron diffraction for ion storage systems. The in situ observation facilitates comprehending real-time ion storage behaviour while the ion storage system is operating, which help us to understand detailed physical and chemical properties. We will discuss how the tools have been used to reveal detailed reaction mechanisms and underlying properties of electrode materials.

Study on the Electrochemical Reaction Mechanism of ZnFe2O4 by In Situ Transmission Electron Microscopy

A family of mixed transition–metal oxides (MTMOs) has great potential for applications as anodes for lithium ion batteries (LIBs). However, the reaction mechanism of MTMOs anodes during lithiation/delithiation is remain unclear. Here, the lithiation/delithiation processes of ZnFe₂O₄ nanoparticles are observed dynamically using in situ transmission electron microscopy (TEM). Our results suggest that during the first lithiation process the ZnFe₂O₄ nanoparticles undergo a conversion process and generate a composite structure of 1–3 nm Fe and Zn nanograins within Li₂O matrix. During the delithiation process, volume contraction and the conversion of Zn and Fe take place with the disappearance of Li₂O, followed by the complete conversion to Fe₂O₃ and ZnO not the original phase ZnFe₂O₄. The following cycles are dominated by the full reversible phase conversion between Zn, Fe and ZnO, Fe₂O₃. The Fe valence evolution during cycles evidenced by electron energy–loss spectroscopy (EELS) techniques also exhibit the reversible conversion between Fe and Fe₂O₃ after the first lithiation, agreeing well with the in situ TEM results. Such in situ TEM observations provide valuable phenomenological insights into electrochemical reaction of MTMOs, which may help to optimize the composition of anode materials for further improved electrochemical performance.

Nanoscience Supporting the Research on the Negative Electrodes of Li-Ion Batteries

Many efforts are currently made to increase the limited capacity of Li-ion batteries using carbonaceous anodes. The way to reach this goal is to move to nano-structured material because the larger surface to volume ratio of particles and the reduction of the electron and Li path length implies a larger specific capacity. Additionally, nano-particles can accommodate such a dilatation/contraction during cycling, resulting in a calendar life compatible with a commercial use. In this review attention is focused on carbon, silicon, and Li₄Ti₅O₁₂ materials, because they are the most promising for applications.

Scanning MWCNT-Nanopipette and Probe Microscopy: Li Patterning and Transport Studies

A carbon-nanotube-enabling scanning probe technique/nanotechnology for manipulating and measuring lithium at the nano/mesoscale is introduced. Scanning Li-nanopipette and probe microscopy (SLi-NPM) is based on a conductive atomic force microscope (AFM) cantilever with an open-ended multi-walled carbon nanotube (MWCNT) affixed to its apex. SLi-NPM operation is demonstrated with a model system consisting of a Li thin film on a Si(111) substrate. By control of bias, separation distance, and contact time, attograms of Li can be controllably pipetted to or from the MWCNT tip. Patterned surface Li features are then directly probed via noncontact AFM measurements with the MWCNT tip. The subsequent decay of Li features is simulated with a mesoscale continuum model, developed here. The Li surface diffusion coefficient for a four (two) Li layer thick film is measured as D=8(±1.2)×10(-15) cm(2) s(-1) (D=1.75(±0.15)×10(-15) cm(2) s(-1)). Dual-Li pipetting/measuring with SLi-NPM enables a broad range of time-dependent Li and nanoelectrode characterization studies of fundamental importance to energy-storage research.

Dynamic In-Situ Experimentation on Nanomaterials at the Atomic Scale

With the development of in situ techniques inside transmission electron microscopes (TEMs), external fields and probes can be applied to the specimen. This development transforms the TEM specimen chamber into a nanolab, in which reactions, structures, and properties can be activated or altered at the nanoscale, and all processes can be simultaneously recorded in real time with atomic resolution. Consequently, the capabilities of TEM are extended beyond static structural characterization to the dynamic observation of the changes in specimen structures or properties in response to environmental stimuli. This extension introduces new possibilities for understanding the relationships between structures, unique properties, and functions of nanomaterials at the atomic scale. Based on the idea of setting up a nanolab inside a TEM, tactics for design of in situ experiments inside the machine, as well as corresponding examples in nanomaterial research, including in situ growth, nanofabrication with atomic precision, in situ property characterization, and nanodevice construction are presented.

Lithiation of silicon nanoparticles confined in carbon nanotubes

Silicon has the highest theoretical lithium storage capacity of all materials at 4200 mAh/g; therefore, it is considered to be a promising candidate as the anode of high-energy-density lithium-ion batteries (LIBs). However, serious volume changes caused by lithium insertion/deinsertion lead to a rapid decay of the performance of the Si anode. Here, a Si nanoparticle (NP)-filled carbon nanotube (CNT) material was prepared by chemical vapor deposition, and a nanobattery was constructed inside a transmission electron microscope (TEM) using the Si NP-filled CNT as working electrode to directly investigate the structural change of the Si NPs and the confinement effect of the CNT during the lithiation and delithiation processes. It is found that the volume expansion (∼180%) of the lithiated Si NPs is restricted by the wall of the CNTs and that the CNT can accommodate this volume expansion without breaking its tubular structure. The Si NP-filled CNTs showed a high reversible lithium storage capacity and desirable high rate capability, because the pulverization and exfoliation of the Si NPs confined in CNTs were efficiently prevented. Our results demonstrate that filling CNTs with high-capacity active materials is a feasible way to make high-performance LIB electrode materials, taking advantage of the unique confinement effect and good electrical conductivity of the CNTs.

Surface coating mediated swelling and fracture of silicon nanowires during lithiation

Surface passivation of silicon anodes is an appealing design strategy for the development of reliable, high-capacity lithium-ion batteries. However, the structural stability of the coating layer and its influence on the lithiation process remain largely unclear. Herein, we show that surface coating mediates the swelling dynamics and the fracture pattern during initial lithiation of crystalline silicon nanopillars. We choose conformally nickel coated silicon architectures as a model system. Experimental findings are interpreted based on a chemomechanical model. Markedly different swelling and fracture regimes have been identified, depending on the coating thickness and silicon nanopillar diameter. Nanopillars with relatively thin coating display anisotropic swelling similar to pristine nanopillars, but with different preferred fracture sites. As the coating thickness increases, the mechanisms become isotropic, with one randomly oriented longitudinal crack that unzips the core-shell structure. The morphology of cracked pillars resembles that of a thin-film electrode on a substrate, which is more amenable to cyclic lithiation without fracture. The knowledge provided here helps clarify the cycling results of coated nanosilicon electrodes and further suggests design rules for better performance electrodes through proper control of the lithiation and fracture.

Bending-induced symmetry breaking of lithiation in germanium nanowires

From signal transduction of living cells to oxidation and corrosion of metals, mechanical stress intimately couples with chemical reactions, regulating these biological and physiochemical processes. The coupled effect is particularly evident in the electrochemical lithiation/delithiation cycling of high-capacity electrodes, such as silicon (Si), where on the one hand lithiation-generated stress mediates lithiation kinetics and on the other the electrochemical reaction rate regulates stress generation and mechanical failure of the electrodes. Here we report for the first time the evidence on the controlled lithiation in germanium nanowires (GeNWs) through external bending. Contrary to the symmetric core-shell lithiation in free-standing GeNWs, we show bending the GeNWs breaks the lithiation symmetry, speeding up lithaition at the tensile side while slowing down at the compressive side of the GeNWs. The bending-induced symmetry breaking of lithiation in GeNWs is further corroborated by chemomechanical modeling. In the light of the coupled effect between lithiation kinetics and mechanical stress in the electrochemical cycling, our findings shed light on strain/stress engineering of durable high-rate electrodes and energy harvesting through mechanical motion.

New insight in understanding oxygen reduction and evolution in solid-state lithium-oxygen batteries using an in situ environmental scanning electron microscope

Via designing a facile microscale all-solid-state lithium-oxygen battery system constructed in an environmental scanning electron microscope, direct visualization of discharge and charge processes of the lithium-oxygen battery is achieved. Different morphologies of the discharge product are observed, including a sphere, conformal film, and red-blood-cell-like shape, with a particle size up to 1.5 μm; whereas upon charge, the decomposition initiates at their surface and continues along a certain direction, instead of from the contact point at the electrode. These new findings indicate that the electron and lithium ion conductivities of Li2O2 could support the growth and decomposition of the discharge product in our system. In addition, our results indicate that various morphologies of Li2O2 arise from the different current density and surface chemistry of CNT, and the growth and decomposition of the particle are related to the uneven distribution of the ionic and electronic conductivities of Li2O2.

In situ transmission electron microscopy study of electrochemical sodiation and potassiation of carbon nanofibers

Carbonaceous materials have great potential for applications as anodes of alkali-metal ion batteries, such as Na-ion batteries and K-ion batteries (NIB and KIBs). We conduct an in situ study of the electrochemically driven sodiation and potassiation of individual carbon nanofibers (CNFs) by transmission electron microscopy (TEM). The CNFs are hollow and consist of a bilayer wall with an outer layer of disordered-carbon (d-C) enclosing an inner layer of crystalline-carbon (c-C). The d-C exhibits about three times volume expansion of the c-C after full sodiation or potassiation, thus suggesting a much higher storage capacity of Na or K ions in d-C than c-C. For the bilayer CNF-based electrode, a steady sodium capacity of 245 mAh/g is measured with a Coulombic efficiency approaching 98% after a few initial cycles. The in situ TEM experiments also reveal the mechanical degradation of CNFs through formation of longitudinal cracks near the c-C/d-C interface during sodiation and potassiation. Geometrical changes of the tube are explained by a chemomechanical model using the anisotropic sodiation/potassiation strains in c-C and d-C. Our results provide mechanistic insights into the electrochemical reaction, microstructure evolution and mechanical degradation of carbon-based anodes during sodiation and potassiation, shedding light onto the development of carbon-based electrodes for NIBs and KIBs.

In situ transmission electron microscopy observation of electrochemical sodiation of individual Co₉S₈-filled carbon nanotubes

The comprehension of fundamental electrochemical behavior and sodiation mechanism is critical for the design of high-performance electrode materials for sodium-ion (Na-ion) batteries. In this paper, the electrochemical sodiation process and microstructure evolution of individual Co9S8-filled carbon nanotubes (CNTs) have been directly visualized and studied using in situ transmission electron microscopy. Upon the first sodiation, a reaction front propagates progressively along the filling nanowire, causing the filled CNT to inflate. The filled CNTs behave differently depending on their structures and the magnitude of the sodiation voltage. For a Co9S8-filled CNT with an open end, the sodiated Co9S8 filler shows a substantial axial elongation of 120.8% and a small radial swelling due to the extrusion of CNT walls. In contrast, the closed CNT shows a major radial expansion of 40.6% and a small axial elongation because of the mechanical confinement of the carbon shells. After sodiation, the spacing between the carbon shells increases from 3.4 to 3.8 Å due to the Na(+)-ion insertion and the single-crystalline Co9S8 filler converts to numerous Co nanograins dispersed in a Na2S matrix. Compared with the gentle microstructure evolution of the CNT under small charging voltage, a strong electrochemical reaction accompanying drastic swelling and fracturing of CNT shells is observed for the CNT electrode under large charging voltage. Our observations provide direct evidence and important insights for the electrochemical process of CNT-based composite materials in Na-ion batteries.

In situ transmission electron microscopy observation of electrochemical behavior of CoS(2) in lithium-ion battery

Metal sulfides are a type of potential anode materials for lithium-ion batteries (LIBs). However, their electrochemical behaviors and mechanism during the charge and discharge process remain unclear. In the present paper, we use CoS2 as a model material to investigate their electrochemical process using in situ transmission electron microscopy (TEM). Two kinds of reaction behaviors are revealed. The pure CoS2 particles show a side-to-side conversion process, in which large and anisotropic size expansion (47.1%) occurs that results in the formation of cracks and fractures in CoS2 particles. In contrast, the CoS2 particles anchored on reduced graphene oxide (rGO) sheets exhibit a core-shell conversion process involving small and homogeneous size expansion (28.6%) and few fractures, which attributes to the excellent Li(+) conductivity of rGO sheets and accounts for the improved cyclability. Single-crystalline CoS2 particle converts to Co nanocrystals of 1-2 nm embedded within Li2S matrix after the first lithiation. The subsequent electrochemical reaction is a reversible phase conversion between Co/Li2S and CoS2 nanocrystals. Our direct observations provide important mechanistic insight for developing high-performance conversion electrodes for LIBs.

In situ transmission electron microscopy investigation of the electrochemical lithiation-delithiation of individual Co9S8/Co-filled carbon nanotubes

Carbon nanotube (CNT)-encapsulated metal sulfides/oxides are promising candidates for application as anode materials in lithium ion battery (LIB), while their electrochemical behavior and mechanism still remain unclear. A comprehensive understanding of the lithiation mechanism at nanoscale of this type of composites will benefit the design and development of high-performance LIB materials. Here, we use Co9S8/Co nanowire-filled CNTs as a model material to investigate the lithium storage mechanism by in situ transmission electron microscopy. For a Co9S8/Co nanowire-filled closed CNT, the reaction front propagates progressively during lithiation, causing an axial elongation of 4.5% and a radial expansion of 32.4%, while the lithiated nanowire core is still confined inside the CNT. Contrastingly, for an open CNT, the lithiated Co9S8 nanowire shows an axial elongation of 94.2% and is extruded out from the open CNT. In particular, a thin graphite shell is drawn out from the CNT wall by the extruded lithiated Co9S8. The thin graphite shell confines the extruded filler and protects the filler from pulverization in the following lithiation-delithiation cycles. During multiple cycles, the Co segment remains intact while the Co9S8 exhibits a reversible transformation between Co9S8 and Co nanograins. Our observations provide direct electrochemical behavior and mechanism that govern the CNT-based anode performance in LIBs.

Nanovoid formation and annihilation in gallium nanodroplets under lithiation-delithiation cycling

The irreversible chemomechanical degradation is a critical issue in the development of high-capacity electrode materials for the next-generation lithium (Li)-ion batteries. Here we report the self-healing behavior of gallium nanodroplets (GaNDs) under electrochemical cycling at room temperature, observed with in situ transmission electron microscopy (TEM). During lithiation, the GaNDs underwent a liquid-to-solid phase transition, forming a crystalline phase (LixGa) with ~160% volume expansion. Owing to the uneven Li flow during lithiation, the fully lithiated GaNDs exhibited highly distorted morphologies. Upon delithiation, the reverse phase transition occurred, accompanied with the nucleation and growth of a nanosized void. After the GaNDs were fully delithiated, the nanovoid gradually annihilated. Our analysis, along with phase field modeling and experimental measurements of the void growth and annihilation, provides mechanistic insights into the void formation and annihilation mechanism. The GaNDs may function as an effective healing agent in durable composite electrodes for high-performance Li-ion batteries, wherein active components, such as Si, are susceptible to fracture.

Atomic-scale observation of lithiation reaction front in nanoscale SnO2 materials

In the present work, taking advantage of aberration-corrected scanning transmission electron microscopy, we show that the dynamic lithiation process of anode materials can be revealed in an unprecedented resolution. Atomically resolved imaging of the lithiation process in SnO2 nanowires illustrated that the movement, reaction, and generation of b = [1[overline]1[overline]1] mixed dislocations leading the lithiated stripes effectively facilitated lithium-ion insertion into the crystalline interior. The geometric phase analysis and density functional theory simulations indicated that lithium ions initial preference to diffuse along the [001] direction in the {200} planes of SnO2 nanowires introduced the lattice expansion and such dislocation behaviors. At the later stages of lithiation, the Li-induced amorphization of rutile SnO2 and the formation of crystalline Sn and LixSn particles in the Li2O matrix were observed.

In situ transmission electron microscopy study of electrochemical lithiation and delithiation cycling of the conversion anode RuO2

Conversion-type electrodes represent a broad class of materials with a new Li(+) reactivity concept. Of these materials, RuO2 can be considered a model material due to its metallic-like conductivity and its high theoretical capacity of 806 mAh/g. In this paper, we use in situ transmission electron microscopy to study the reaction between single-crystal RuO2 nanowires and Li(+). We show that a large volume expansion of 95% occurs after lithiation, 26% of which is irreversible after delithiation. Significant surface roughening and lithium embrittlement are also present. Furthermore, we show that the initial reaction from crystalline RuO2 to the fully lithiated mixed phase of Ru/Li2O is not fully reversible, passing through an intermediate phase of LixRuO2. In subsequent cycles, the phase transitions are between amorphous RuO2 in the delithiated state and a nanostructured network of Ru/Li2O in the fully lithiated phase.

Visualizing the electrochemical reaction of ZnO nanoparticles with lithium by in situ TEM: two reaction modes are revealed

The lithiation reaction of ZnO as an anode in a lithium-ion battery (LIB) is unclear. The electrochemical behavior of ZnO was investigated inside a transmission electron microscope (TEM) by constructing a nano-LIB using an individual ZnO/graphene sheet as the electrode. The lithiation reaction of ZnO/graphene was monitored by simultaneous determination of the structure with high-resolution TEM, electron diffraction pattern and electron energy-loss spectroscopy. Two kinds of reaction modes were revealed in terms of different reaction rates. One was the violent reaction mode, in which one particle can evolve into an aggregate of many nanoparticles within the Li2O matrix in 1-2 min. The other was the peaceful evolution mode, in which each ZnO nanoparticle evolves into a core-shell particle with multi-domains constituted of Zn and LiZn nanograins. Abnormally large Zn nanocrystals grow quickly in the violent reaction mode, which can suppress the formation of LiZn and impair the reversible capacity. Our observations give direct evidence and important insights for the lithiation mechanism of metal oxide anodes in LIBs.

In situ transmission electron microscopy observations of electrochemical oxidation of Li2O2

In this Letter, we report the first in situ transmission electron microscopy observation of electrochemical oxidation of Li2O2, providing insights into the rate limiting processes that govern charge in Li-O2 cells. In these studies, oxidation of electrochemically formed Li2O2 particles, supported on multiwall carbon nanotutubes (MWCNTs), was found to occur preferentially at the MWCNT/Li2O2 interface, suggesting that electron transport in Li2O2 ultimately limits the oxidation kinetics at high rates or overpotentials.

Tough germanium nanoparticles under electrochemical cycling

Mechanical degradation of the electrode materials during electrochemical cycling remains a serious issue that critically limits the capacity retention and cyclability of rechargeable lithium-ion batteries. Here we report the highly reversible expansion and contraction of germanium nanoparticles under lithiation-delithiation cycling with in situ transmission electron microscopy (TEM). During multiple cycles to the full capacity, the germanium nanoparticles remained robust without any visible cracking despite ∼260% volume changes, in contrast to the size-dependent fracture of silicon nanoparticles upon the first lithiation. The comparative in situ TEM study of fragile silicon nanoparticles suggests that the tough behavior of germanium nanoparticles can be attributed to the weak anisotropy of the lithiation strain at the reaction front. The tough germanium nanoparticles offer substantial potential for the development of durable, high-capacity, and high-rate anodes for advanced lithium-ion batteries.

Microstructural evolution of tin nanoparticles during in situ sodium insertion and extraction

The microstructural changes and phase transformations of tin nanoparticles during electrochemical sodiation were studied with a nanosized sodium ion battery using in situ transmission electron microscopy. It was found that the first sodiation process occurred in two steps; that is, the crystalline Sn nanoparticles were initially sodiated via a two-phase mechanism with a migrating phase boundary to form a Na-poor, amorphous Na(x)Sn alloy (x ~ 0.5), which was further sodiated to several Na-rich amorphous phases and finally to the crystallized Na(15)Sn(4) (x = 3.75) via a single-phase mechanism. The volumetric expansion was about 60% in the first step and 420% after the second step. However, despite the huge expansion, cracking or fracture was not observed, which is attributed to the second step of the single-phase sodiation that accommodates large portion of the sodiation-induced stress over the entire particle. Excellent cyclability was also observed during the reversible sodiation/desodiation cycles, showing great potential of Sn nanoparticles as a robust electrode material for rechargeable batteries.

Sandwich-lithiation and longitudinal crack in amorphous silicon coated on carbon nanofibers

Silicon-carbon nanofibers coaxial sponge, with strong mechanical integrity and improved electronic conductivity, is a promising anode structure to apply into commercial high-capacity lithium ion batteries. We characterized the electrochemical and mechanical behaviors of amorphous silicon-coated carbon nanofibers (a-Si/CNFs) with in situ transmission electron microscopy (TEM). It was found that lithiation of the a-Si coating layer occurred from the surface and the a-Si/CNF interface concurrently, and propagated toward the center of the a-Si layer. Such a process leads to a sandwiched Li(x)Si/Si/Li(x)Si structure, indicating fast Li transport through the a-Si/CNF interface. Nanocracks and sponge-like structures developed in the a-Si layer during the lithiation-delithiation cycles. Lithiation of the a-Si layer sealed in the hollow CNF was also observed, but at a much lower speed than the counterpart of the a-Si layer coated on the CNF surface. An analytical solution of the stress field was formulated based on the continuum theory of finite deformation, explaining the experimental observation of longitudinal crack formation and general mechanical degradation mechanism in a-Si/CNF electrode.

In situ TEM study of lithiation behavior of silicon nanoparticles attached to and embedded in a carbon matrix

Rational design of silicon and carbon nanocomposite with a special topological feature has been demonstrated to be a feasible way for mitigating the capacity fading associated with the large volume change of silicon anode in lithium ion batteries. Although the lithiation behavior of silicon and carbon as individual components has been well understood, lithium ion transport behavior across a network of silicon and carbon is still lacking. In this paper, we probe the lithiation behavior of silicon nanoparticles attached to and embedded in a carbon nanofiber using in situ TEM and continuum mechanical calculation. We found that aggregated silicon nanoparticles show contact flattening upon initial lithiation, which is characteristically analogous to the classic sintering of powder particles by a neck-growth mechanism. As compared with the surface-attached silicon particles, particles embedded in the carbon matrix show delayed lithiation. Depending on the strength of the carbon matrix, lithiation of the embedded silicon nanoparticles can lead to the fracture of the carbon fiber. These observations provide insights on lithium ion transport in the network-structured composite of silicon and carbon and ultimately provide fundamental guidance for mitigating the failure of batteries due to the large volume change of silicon anodes.

Emerging applications of atomic layer deposition for lithium-ion battery studies

Lithium-ion batteries (LIBs) are used widely in today's consumer electronics and offer great potential for hybrid electric vehicles (HEVs), plug-in HEVs, pure EVs, and also in smart grids as future energy-storage devices. However, many challenges must be addressed before these future applications of LIBs are realized, such as the energy and power density of LIBs, their cycle and calendar life, safety characteristics, and costs. Recently, a technique called atomic layer deposition (ALD) attracted great interest as a novel tool and approach for resolving these issues. In this article, recent advances in using ALD for LIB studies are thoroughly reviewed, covering two technical routes: 1) ALD for designing and synthesizing new LIB components, i.e., anodes, cathodes, and solid electrolytes, and; 2) ALD used in modifying electrode properties via surface coating. This review will hopefully stimulate more extensive and insightful studies on using ALD for developing high-performance LIBs.

In situ TEM investigation of congruent phase transition and structural evolution of nanostructured silicon/carbon anode for lithium ion batteries

It is well-known that upon lithiation, both crystalline and amorphous Si transform to an armorphous Li(x)Si phase, which subsequently crystallizes to a (Li, Si) crystalline compound, either Li(15)Si(4) or Li(22)Si(5). Presently, the detailed atomistic mechanism of this phase transformation and the degradation process in nanostructured Si are not fully understood. Here, we report the phase transformation characteristic and microstructural evolution of a specially designed amorphous silicon (a-Si) coated carbon nanofiber (CNF) composite during the charge/discharge process using in situ transmission electron microscopy and density function theory molecular dynamic calculation. We found the crystallization of Li(15)Si(4) from amorphous Li(x)Si is a spontaneous, congruent phase transition process without phase separation or large-scale atomic motion, which is drastically different from what is expected from a classic nucleation and growth process. The a-Si layer is strongly bonded to the CNF and no spallation or cracking is observed during the early stages of cyclic charge/discharge. Reversible volume expansion/contraction upon charge/discharge is fully accommodated along the radial direction. However, with progressive cycling, damage in the form of surface roughness was gradually accumulated on the coating layer, which is believed to be the mechanism for the eventual capacity fade of the composite anode during long-term charge/discharge cycling.

Lithium-assisted electrochemical welding in silicon nanowire battery electrodes

From in situ transmission electron microscopy (TEM) observations, we present direct evidence of lithium-assisted welding between physically contacted silicon nanowires (SiNWs) induced by electrochemical lithiation and delithiation. This electrochemical weld between two SiNWs demonstrates facile transport of lithium ions and electrons across the interface. From our in situ observations, we estimate the shear strength of the welded region after delithiation to be approximately 200 MPa, indicating that a strong bond is formed at the junction of two SiNWs. This welding phenomenon could help address the issue of capacity fade in nanostructured silicon battery electrodes, which is typically caused by fracture and detachment of active materials from the current collector. The process could provide for more robust battery performance either through self-healing of fractured components that remain in contact or through the formation of a multiconnected network architecture.