Despite remarkable progress, lithium ion batteries still need higher energy density and better cy... more Despite remarkable progress, lithium ion batteries still need higher energy density and better cycle life for consumer electronics, electric drive vehicles and large-scale renewable energy storage applications. Silicon has recently been explored as a promising anode material for high energy batteries; however, attaining long cycle life remains a significant challenge due to materials pulverization during cycling and an unstable solid-electrolyte interphase. Here, we report double-walled silicon nanotube electrodes that can cycle over 6000 times while retaining more than 85 % of the initial capacity. This excellent performance is due to the unique double-walled structure in which the outer silicon oxide wall confines the inner silicon wall to expand only inward during lithiation, resulting in a stable solid-electrolyte interphase. This structural concept is general and could be extended to other battery materials that undergo large volume changes. Among all the rechargeable battery t...
Abstract Recent experiments in micron- or sub-micron metallic pillars have displayed size depende... more Abstract Recent experiments in micron- or sub-micron metallic pillars have displayed size dependent plasticity. Dislocation dynamics (DD) simulations can be a useful tool in studying dislocation-mediated plasticity, which is widely believed to contribute to observed size effects at small length scale. In particular, the plasticity in body-centered cubic (BCC) metals is known to have a strong temperature dependence, which stems from the thermally activated motion of screw dislocations. In this study, we develop a DD model based on the atomistic characterization of dislocation mobility and potential source mechanisms to investigate the temperature dependent plasticity in BCC micropillars. Our models show the dislocation source mechanism changes with respect to temperature due to the change in mobility of screw dislocations, and these results are compared with experimental results and theoretical models. In addition, the size dependence increases with temperature, which agrees with recent experimental observations.
Abstract At the continuum length scale, mechanical properties of metals show relatively weak orie... more Abstract At the continuum length scale, mechanical properties of metals show relatively weak orientation dependence; however, they exhibit strong anisotropic behaviors as the size of sample decreases to micron and nanometer length scales. In this study, three-dimensional dislocation dynamics (DD) simulations are performed to investigate the orientation-dependent plasticity in submicron face-centered cubic (FCC) micropillars subjected to torsion. Accommodating results from atomistic modeling, updated surface nucleation schemes in DD models have been developed for three orientations ([001], [101], and [111]), allowing investigation of the dislocation microstructure evolution and the corresponding anisotropic mechanical response upon torsional loading and unloading. The DD simulation results show that the coaxial and hexagonal networks formed in [101] and [111] oriented nanopillars, respectively, exhibited excellent plastic recovery, while the rectangular network formed in the [001] crystal orientation was more stable and did not experience as much plastic recovery.
The plasticity of body‐centered cubic (bcc) metals is dependent of temperature as well as sample ... more The plasticity of body‐centered cubic (bcc) metals is dependent of temperature as well as sample dimension at the micrometer scale, but the effects of cryogenic temperature on the plasticity and the related failure process in micron‐sized bcc metals have not been studied under uniaxial tension. In this work, we utilized in situ cryogenic micro‐tensile tests, transmission electron microscopy, and dislocation dynamic simulations to examine the plasticity and failure processes of [001]‐oriented bcc niobium micropillars. Our study reveals that a strong suppression of cross‐slip at low temperatures prevents dislocation multiplication and leads to a dislocation starvation state, at which no mobile dislocation exists due to the rapid annihilation of dislocations at free surfaces. New dislocations are then nucleated until stress concentration at a slip step creates a micro‐crack, the propagation of which leads to catastrophic failure. This unique failure process results from the combined effects of sample dimension and temperature.
Si is an attractive negative electrode material for lithium ion batteries due to its high specifi... more Si is an attractive negative electrode material for lithium ion batteries due to its high specific capacity (≈3600 mAh g–1). However, the huge volume swelling and shrinking during cycling, which mimics a breathing effect at the material/electrode/cell level, leads to several coupled issues including fracture of Si particles, unstable solid electrolyte interphase, and low Coulombic efficiency. In this work, the regulation of the breathing effect is reported by using Si–C yolk–shell nanocomposite which has been well‐developed by other researchers. The focus is on understanding how the nanoscaled materials design impacts the mechanical and electrochemical response at electrode level. For the first time, it is possible to observe one order of magnitude of reduction on breathing effect at the electrode level during cycling: the electrode thickness variation reduced down to 10%, comparing with 100% in the electrode with Si nanoparticles as active materials. The Si–C yolk–shell nanocomposi...
Following an explosion of studies of silicon as a negative electrode for Li-ion batteries, the an... more Following an explosion of studies of silicon as a negative electrode for Li-ion batteries, the anomalous volumetric changes and fracture of lithiated single Si particles have attracted significant attention in various fields, including mechanics. However, in real batteries, lithiation occurs simultaneously in clusters of Si in a confined medium. Hence, understanding how the individual Si structures interact during lithiation in a closed space is necessary. Here, we demonstrate physical and mechanical interactions of swelling Si structures during lithiation using well-defined Si nanopillar pairs. Ex situ SEM and in situ TEM studies reveal that compressive stresses change the reaction kinetics so that preferential lithiation occurs at free surfaces when the pillars are mechanically clamped. Such mechanical interactions enhance the fracture resistance of lithiated Si by lessening the tensile stress concentrations in Si structures. This study will contribute to improved design of Si str...
Despite remarkable progress, lithium ion batteries still need higher energy density and better cy... more Despite remarkable progress, lithium ion batteries still need higher energy density and better cycle life for consumer electronics, electric drive vehicles and large-scale renewable energy storage applications. Silicon has recently been explored as a promising anode material for high energy batteries; however, attaining long cycle life remains a significant challenge due to materials pulverization during cycling and an unstable solid-electrolyte interphase. Here, we report double-walled silicon nanotube electrodes that can cycle over 6000 times while retaining more than 85 % of the initial capacity. This excellent performance is due to the unique double-walled structure in which the outer silicon oxide wall confines the inner silicon wall to expand only inward during lithiation, resulting in a stable solid-electrolyte interphase. This structural concept is general and could be extended to other battery materials that undergo large volume changes. Among all the rechargeable battery t...
Abstract Recent experiments in micron- or sub-micron metallic pillars have displayed size depende... more Abstract Recent experiments in micron- or sub-micron metallic pillars have displayed size dependent plasticity. Dislocation dynamics (DD) simulations can be a useful tool in studying dislocation-mediated plasticity, which is widely believed to contribute to observed size effects at small length scale. In particular, the plasticity in body-centered cubic (BCC) metals is known to have a strong temperature dependence, which stems from the thermally activated motion of screw dislocations. In this study, we develop a DD model based on the atomistic characterization of dislocation mobility and potential source mechanisms to investigate the temperature dependent plasticity in BCC micropillars. Our models show the dislocation source mechanism changes with respect to temperature due to the change in mobility of screw dislocations, and these results are compared with experimental results and theoretical models. In addition, the size dependence increases with temperature, which agrees with recent experimental observations.
Abstract At the continuum length scale, mechanical properties of metals show relatively weak orie... more Abstract At the continuum length scale, mechanical properties of metals show relatively weak orientation dependence; however, they exhibit strong anisotropic behaviors as the size of sample decreases to micron and nanometer length scales. In this study, three-dimensional dislocation dynamics (DD) simulations are performed to investigate the orientation-dependent plasticity in submicron face-centered cubic (FCC) micropillars subjected to torsion. Accommodating results from atomistic modeling, updated surface nucleation schemes in DD models have been developed for three orientations ([001], [101], and [111]), allowing investigation of the dislocation microstructure evolution and the corresponding anisotropic mechanical response upon torsional loading and unloading. The DD simulation results show that the coaxial and hexagonal networks formed in [101] and [111] oriented nanopillars, respectively, exhibited excellent plastic recovery, while the rectangular network formed in the [001] crystal orientation was more stable and did not experience as much plastic recovery.
The plasticity of body‐centered cubic (bcc) metals is dependent of temperature as well as sample ... more The plasticity of body‐centered cubic (bcc) metals is dependent of temperature as well as sample dimension at the micrometer scale, but the effects of cryogenic temperature on the plasticity and the related failure process in micron‐sized bcc metals have not been studied under uniaxial tension. In this work, we utilized in situ cryogenic micro‐tensile tests, transmission electron microscopy, and dislocation dynamic simulations to examine the plasticity and failure processes of [001]‐oriented bcc niobium micropillars. Our study reveals that a strong suppression of cross‐slip at low temperatures prevents dislocation multiplication and leads to a dislocation starvation state, at which no mobile dislocation exists due to the rapid annihilation of dislocations at free surfaces. New dislocations are then nucleated until stress concentration at a slip step creates a micro‐crack, the propagation of which leads to catastrophic failure. This unique failure process results from the combined effects of sample dimension and temperature.
Si is an attractive negative electrode material for lithium ion batteries due to its high specifi... more Si is an attractive negative electrode material for lithium ion batteries due to its high specific capacity (≈3600 mAh g–1). However, the huge volume swelling and shrinking during cycling, which mimics a breathing effect at the material/electrode/cell level, leads to several coupled issues including fracture of Si particles, unstable solid electrolyte interphase, and low Coulombic efficiency. In this work, the regulation of the breathing effect is reported by using Si–C yolk–shell nanocomposite which has been well‐developed by other researchers. The focus is on understanding how the nanoscaled materials design impacts the mechanical and electrochemical response at electrode level. For the first time, it is possible to observe one order of magnitude of reduction on breathing effect at the electrode level during cycling: the electrode thickness variation reduced down to 10%, comparing with 100% in the electrode with Si nanoparticles as active materials. The Si–C yolk–shell nanocomposi...
Following an explosion of studies of silicon as a negative electrode for Li-ion batteries, the an... more Following an explosion of studies of silicon as a negative electrode for Li-ion batteries, the anomalous volumetric changes and fracture of lithiated single Si particles have attracted significant attention in various fields, including mechanics. However, in real batteries, lithiation occurs simultaneously in clusters of Si in a confined medium. Hence, understanding how the individual Si structures interact during lithiation in a closed space is necessary. Here, we demonstrate physical and mechanical interactions of swelling Si structures during lithiation using well-defined Si nanopillar pairs. Ex situ SEM and in situ TEM studies reveal that compressive stresses change the reaction kinetics so that preferential lithiation occurs at free surfaces when the pillars are mechanically clamped. Such mechanical interactions enhance the fracture resistance of lithiated Si by lessening the tensile stress concentrations in Si structures. This study will contribute to improved design of Si str...
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