Understanding the electrochemical reaction of lithium with single silicon nanostructures [electronic resource]
Keywords: Materials Science and Engineering.

Lithium-ion batteries are commonly used for mobile devices, but improvements in energy density, cycle life, and cost are urgently needed to enable emerging applications such as electric vehicles and grid storage. One way to increase energy density is to develop new high-capacity electrode materials. Silicon is an interesting negative electrode material because it has an order of magnitude higher theoretical specific capacity than conventional anodes. However, silicon and other novel materials exhibit different reaction and degradation mechanisms than traditional battery materials; in particular, silicon undergoes 300% volume expansion upon lithiation, which can lead to fracture and mechanical degradation. For stable electrochemical cycling, it is vital to understand these processes in detail. Herein, novel in-situ and ex-situ electron microscopy techniques are used to study the electrochemical reaction mechanisms of lithium with individual silicon nanostructures. The experiments reveal the surprising effects of nanostructure shape, size, crystallography, and crystallinity on volume changes, fracture, and reaction kinetics, and the results provide guidelines for design of better electrodes. In the first part of this dissertation, the lithiation and delithiation mechanisms of crystalline Si (c-Si) nanostructures are presented. Lithiation occurs via a two-phase reaction with an atomically sharp reaction front between the crystalline silicon and an amorphous LixSi phase. Late in the lithiation process, the amorphous LixSi is transformed to the crystalline Li15Si4 phase, which is confirmed to be the terminal lithiated phase. Interestingly, lithiation and volume expansion are found to be highly anisotropic, with preferential lithiation at {110} crystallographic planes of silicon. This phenomenon is caused by differing reaction kinetics at different planes. The lithiation mechanisms of amorphous Si (a-Si) nanostructures are also probed, and lithiation is found to proceed by an unexpected two-phase mechanism in a similar manner as c-Si, although lithiation of a-Si is isotropic because there is no underlying crystallographic symmetry. Next, observations of fracture are presented. Crystalline Si nanoparticles and nanopillars above a certain critical size fracture at the surface upon lithiation. This surface fracture is due to tensile hoop stress that evolves at the surface due to the two-phase reaction mechanism. For pillars, the critical diameter for fracture is between 240 and 360 nm, while for particles it is somewhat lower (150-200 nm). Anisotropic expansion also strongly influences fracture by causing stress intensification at certain locations. Finally, even though a-Si and c-Si are lithiated by a similar two-phase mechanism, a-Si particles up to 800 nm in diameter do not fracture upon lithiation, indicating that a-Si could be more mechanically stable in battery electrodes. The evolution of mechanical stress was also found to influence the kinetics of lithiation in c-Si nanoparticles. Experiments show that the reaction rate slows dramatically during lithiation of c-Si nanoparticles of all sizes; the measured reaction slowing is not consistent with conventional diffusional or reaction front control of the kinetics. Instead, analytical modeling of the hydrostatic stress in particles reveals that the compressive stress is significant enough to influence the thermodynamic driving force for the lithiation reaction, which can stop or slow the reaction. Amorphous Si particles do not display this behavior, indicating dissimilar stress evolution. Finally, the effects of external constraint on volume changes are studied. The results reveal that thin metallic coatings fracture upon volume expansion of the underlying Si, and that thicker coatings could decrease the equilibrium concentration of Li in the Li-Si phase. Also, in-situ results on the lithiation of hollow nanostructures are presented, indicating a promising route for stable cycling performance.