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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.

Electrochemical experiments were conducted on {100}, {110}, and {111} silicon wafers to characterize the kinetics of the initial lithiation of crystalline Si electrodes. Under constant current conditions, we observed constant cell potentials for all orientations, indicating the existence of a phase boundary that separates crystalline silicon from the amorphous lithiated phase. For a given potential, the velocity of this boundary was found to be faster for {110} silicon than for the other two orientations. We show that our measurements of varying phase boundary velocities can accurately account for anisotropic morphologies and fracture developed in crystalline silicon nanopillars. We also present a kinetic model by considering the redox reaction at the electrolyte/lithiated silicon interface, diffusion of lithium through the lithiated phase, and the chemical reaction at the lithiated silicon/crystalline silicon interface. From this model, we quantify the rates of the reactions at the interfaces and estimate a lower bound on the diffusivity through the lithiated silicon phase.

Silicon can host a large amount of lithium, making it a promising electrode for high-capacity lithium-ion batteries. Recent experiments indicate that silicon experiences large plastic deformation upon Li absorption, which can significantly decrease the stresses induced by lithiation and thus mitigate fracture failure of electrodes. These issues become especially relevant in nanostructured electrodes with confined geometries. On the basis of first-principles calculations, we present a study of the microscopic deformation mechanism of lithiated silicon at relatively low Li concentration, which captures the onset of plasticity induced by lithiation. We find that lithium insertion leads to breaking of Si-Si bonds and formation of weaker bonds between neighboring Si and Li atoms, which results in a decrease in Young's modulus, a reduction in strength, and a brittle-to-ductile transition with increasing Li concentration. The microscopic mechanism of large plastic deformation is attributed to continuous lithium-assisted breaking and re-forming of Si-Si bonds and the creation of nanopores.

Silicon can host a large amount of lithium, making it a promising electrode for high-capacity lithium-ion batteries. Upon absorbing lithium, silicon swells several times its volume; the deformation often induces large stresses and pulverizes silicon. Recent experiments, however, indicate that under certain conditions lithiation causes inelastic deformation. This paper models such an inelastic host of lithium by considering diffusion, elastic-plastic deformation, and fracture. The model shows that fracture is averted for a small and soft host-an inelastic host of a small feature size and low yield strength. [ABSTRACT FROM AUTHOR]

Abstract: Of all materials, silicon has the highest capacity to store lithium, and is being developed as an electrode for lithium-ion batteries. Upon absorbing a large amount of lithium, the electrode swells greatly, with a volumetric change up to 300%. The swelling is inevitably constrained in practice, often leading to stress and fracture. Evidence has accumulated that the swelling-induced stress can be partially relieved by plastic flow, and that electrodes of small feature sizes can survive many cycles of lithiation and delithiation without fracture. Here we simulate a particle of an electrode subject to cyclic lithiation and delithiation. A recently developed theory of concurrent large swelling and finite-strain plasticity is used to co-evolve fields of stress, deformation, concentration of lithium, and chemical potential of lithium. We identify three types of behavior. When the yield strength is high and the charging rate is low, the entire particle deforms elastically in all cycles. When the yield strength is low and the charging rate is high, the particle (or part of it) undergoes cyclic plasticity. Under intermediate conditions, the particle exhibits the shakedown behavior: part of the particle flows plastically in a certain number of initial cycles, and then the entire particle remains elastic in subsequent cycles. We discuss the effect of the three types of behavior on the capacity and the electrochemical efficiency. [Copyright &y& Elsevier]

Abstract: In a novel design of lithium-ion batteries, hollow electrode particles coated with stiff shells are used to mitigate mechanical and chemical degradation. In particular, silicon anodes of such core–shell nanostructures have been cycled thousands of times with little capacity fading. To reduce weight and to facilitate lithium diffusion, the shell should be thin. However, to avert fracture and debonding from the core, the shell must be sufficiently thick. This tradeoff is considered here by calculating the stress fields resulting from concurrent insertion reaction and plastic flow for both spherical and cylindrical hollow core–shell nanostructures. Conditions to avert fracture and debonding are identified in terms of the radius of the core, the thickness of the shell, and the state of charge. The effect of the stress on the electrochemical reaction is also discussed. [Copyright &y& Elsevier]

Abstract: Magnesium films of various thicknesses were first deposited on silicon (111) substrates by magnetron sputtering method and then annealed in annealing furnace filled with argon gas. The effects of the magnesium film thickness and the annealing temperature on the formation of Mg2Si films were investigated by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). The Mg2Si thin films thus obtained were found to be polycrystalline and the Mg2Si (220) orientation is preferred regardless of the magnesium film thickness and annealing temperature. XRD results indicate that high quality magnesium silicide films are produced if the magnesium/silicon samples are annealed at 400°C for 5h. Otherwise, the synthesized films annealed at annealing temperatures lower than 350°C or higher than 450°C contain magnesium crystallites or magnesium oxide. SEM images have revealed that microstructure grains in the polycrystalline films are about 1–5μm in dimensions, and the texture of the Mg2Si films becomes denser and more homogeneous as the thickness of the magnesium film increases. [Copyright &y& Elsevier]