14 research outputs found
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<sub><i>x</i></sub>Sn alloy (<i>x</i> ∼ 0.5), which was further sodiated to several Na-rich amorphous
phases and finally to the crystallized Na<sub>15</sub>Sn<sub>4</sub> (<i>x</i> = 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
Size-Dependent Fracture of Silicon Nanoparticles During Lithiation
Lithiation of individual silicon nanoparticles was studied in real time with <i>in situ</i> transmission electron microscopy. A strong size dependence of fracture was discovered; that is, there exists a critical particle diameter of ∼150 nm, below which the particles neither cracked nor fractured upon first lithiation, and above which the particles initially formed surface cracks and then fractured due to lithiation-induced swelling. The unexpected surface cracking arose owing to the buildup of large tensile hoop stress, which reversed the initial compression, in the surface layer. The stress reversal was attributed to the unique mechanism of lithiation in crystalline Si, taking place by movement of a two-phase boundary between the inner core of pristine Si and the outer shell of amorphous Li–Si alloy. While the resulting hoop tension tended to initiate surface cracks, the small-sized nanoparticles nevertheless averted fracture. This is because the stored strain energy from electrochemical reactions was insufficient to drive crack propagation, as dictated by the interplay between the two length scales, that is, particle diameter and crack size, that control the fracture. These results are diametrically opposite to those obtained previously from single-phase modeling, which predicted only compressive hoop stress in the surface layer and thus crack initiation from the center in lithiated Si particles and wires. Our work provides direct evidence of the mechanical robustness of small Si nanoparticles for applications in lithium ion batteries
Self-Limiting Lithiation in Silicon Nanowires
The rates of charging and discharging in lithium-ion batteries (LIBs) are critically controlled by the kinetics of Li insertion and extraction in solid-state electrodes. Silicon is being intensively studied as a high-capacity anode material for LIBs. However, the kinetics of Li reaction and diffusion in Si remain unclear. Here we report a combined experimental and theoretical study of the lithiation kinetics in individual Si nanowires. By using <i>in situ</i> transmission electron microscopy, we measure the rate of growth of a surface layer of amorphous Li<sub><i>x</i></sub>Si in crystalline Si nanowires during the first lithiation. The results show the self-limiting lithiation, which is attributed to the retardation effect of the lithiation-induced stress. Our work provides a direct measurement of the nanoscale growth kinetics in lithiated Si, and has implications on nanostructures for achieving the high capacity and high rate in the development of high performance LIBs
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 <i>in situ</i> 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 <i>in situ</i> 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
Size-Dependent Fracture of Silicon Nanoparticles During Lithiation
Lithiation of individual silicon nanoparticles was studied in real time with <i>in situ</i> transmission electron microscopy. A strong size dependence of fracture was discovered; that is, there exists a critical particle diameter of ∼150 nm, below which the particles neither cracked nor fractured upon first lithiation, and above which the particles initially formed surface cracks and then fractured due to lithiation-induced swelling. The unexpected surface cracking arose owing to the buildup of large tensile hoop stress, which reversed the initial compression, in the surface layer. The stress reversal was attributed to the unique mechanism of lithiation in crystalline Si, taking place by movement of a two-phase boundary between the inner core of pristine Si and the outer shell of amorphous Li–Si alloy. While the resulting hoop tension tended to initiate surface cracks, the small-sized nanoparticles nevertheless averted fracture. This is because the stored strain energy from electrochemical reactions was insufficient to drive crack propagation, as dictated by the interplay between the two length scales, that is, particle diameter and crack size, that control the fracture. These results are diametrically opposite to those obtained previously from single-phase modeling, which predicted only compressive hoop stress in the surface layer and thus crack initiation from the center in lithiated Si particles and wires. Our work provides direct evidence of the mechanical robustness of small Si nanoparticles for applications in lithium ion batteries
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 <i>in situ</i> 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 <i>in situ</i> 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
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 <i>in situ</i> 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 <i>in situ</i> 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
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 <i>in situ</i> 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 <i>in situ</i> 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
Tailoring Lithiation Behavior by Interface and Bandgap Engineering at the Nanoscale
Controlling the transport of lithium
(Li) ions and their reaction
with electrodes is central in the design of Li-ion batteries for achieving
high capacity, high rate, and long lifetime. The flexibility in composition
and structure enabled by tailoring electrodes at the nanoscale could
drastically change the ionic transport and help meet new levels of
Li-ion battery performance. Here, we demonstrate that radial heterostructuring
can completely suppress the commonly observed surface insertion of
Li ions in all reported nanoscale systems to date and to exclusively
induce axial lithiation along the ⟨111⟩ direction in
a layer-by-layer fashion. The new lithiation behavior is achieved
through the deposition of a conformal, epitaxial, and ultrathin silicon
(Si) shell on germanium (Ge) nanowires, which creates an effective
chemical potential barrier for Li ion diffusion through and reaction
at the nanowire surface, allowing only axial lithiation and volume
expansion. These results demonstrate for the first time that interface
and bandgap engineering of electrochemical reactions can be utilized
to control the nanoscale ionic transport/insertion paths and thus
may be a new tool to define the electrochemical reactions in Li-ion
batteries
Tailoring Lithiation Behavior by Interface and Bandgap Engineering at the Nanoscale
Controlling the transport of lithium
(Li) ions and their reaction
with electrodes is central in the design of Li-ion batteries for achieving
high capacity, high rate, and long lifetime. The flexibility in composition
and structure enabled by tailoring electrodes at the nanoscale could
drastically change the ionic transport and help meet new levels of
Li-ion battery performance. Here, we demonstrate that radial heterostructuring
can completely suppress the commonly observed surface insertion of
Li ions in all reported nanoscale systems to date and to exclusively
induce axial lithiation along the ⟨111⟩ direction in
a layer-by-layer fashion. The new lithiation behavior is achieved
through the deposition of a conformal, epitaxial, and ultrathin silicon
(Si) shell on germanium (Ge) nanowires, which creates an effective
chemical potential barrier for Li ion diffusion through and reaction
at the nanowire surface, allowing only axial lithiation and volume
expansion. These results demonstrate for the first time that interface
and bandgap engineering of electrochemical reactions can be utilized
to control the nanoscale ionic transport/insertion paths and thus
may be a new tool to define the electrochemical reactions in Li-ion
batteries