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Size Dependent Fracture of Silicon Nanoparticles During Lithiation

ACS Nano, DOI: 10.1021/nn204476h

Lithiation of individual silicon nanoparticles was studied in real time
with in situ transmission electron microscopy. A strong size dependence
of fracture was discovered, i.e., 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, i.e.,
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.

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