By electrochemically cycling these electrodes vs. Li/Li⁺, crack formation and growth in Si particles during the lithiation/de-lithiation cycles were monitored using analytical microscopy and surface characterization techniques. An interesting self-healing process occurred and consisted of arresting of cracks formed in Si particles by the Al matrix, and closure of cracks during de-lithiation.

—Bhattacharya and Alpas

For their experiments, the team used cylindrical electrodes machined from a cast Al-Si alloy. The volume percent of Si in the electrode was 18.5%. Si particles had a polygonal-shaped morphology with an aspect ratio of 1.9 ± 0.4 and were uniformly dispersed in a ductile Al matrix.

Using an optical microscope with a large depth-of-field, they continuously observed the electrode surfaces during cycling. Microscopic features of the electrode surface such as the cracks, having lengths as small as 1 mm, that were formed on Si particles could be detected and recorded in the time scale of the experiments. In addition, the 1000 magnification objective of the optical microscope was used interchangeably with a 50 objective of a Raman spectrometer to collect data showing changes in the crystal structure at the Si surface during the tests.

They also performed scanning (SEM) and transmission electron microscopy (TEM) observation of the cracks in Si particles.

Among their conclusions were:

• The lithiation of Si was accompanied by phase transformation from crystalline to amorphous in the near-surface regions (depth ~100 nm) of the particles. Compressive stresses in this layer caused local spallation in the form of crevice formation possibly as a result of compressive shear (through-thickness) cracks that propagated at 45˚ to the amorphous/crystalline interface.

• When the stress concentration at the crevice tips exceeded the yield strength of underlying crystalline Si, slip occurred by dislocation movement on the {022} planes of Si. Cracks nucleated from the bottom of the crevices propagated within the Si. The density of cracks increased as the voltage decreased during lithiation.

• The Al matrix surrounding Si particles served to arrest the cracks at the Si/Al interface, which was the first step of the self-healing process. Without the Al matrix with high toughness, the crack propagation would have continued and led to fragmentation of the Si.

• Cracks that propagated in the Si particles acted as diffusion channels for Li and electrolyte reduction products that, in turn, were responsible for the creation of new amorphous zones around the cracks.

• The closure of the lithiation-induced cracks constituted the second stage of the self-healing process during de-lithiation. An increased volume due to local amorphization exerted compressive stresses to the crack edges during de-lithiation.

Schematic representation of electrochemical cycling-induced damage in Si particles in Si-Al anodes. (1) Prior to cycling, the microstructure of the electrode depicted a uniform distribution of the Si particles in the ductile Al matrix. (2) As lithiation occurred, the near-surface regions in Si transformed to amorphous from crystalline structure. Crevices were formed in the amorphous layer due to propagation of compressive shear cracks. The crevices also acted as stress concentrators to initiate the cracks in the crystalline interior. The crack surfaces provided easy diffusion paths for the electrolyte reduction products (Li and Cl) that were deposited on the crack flanks. However, the surrounding Al matrix acted as an impediment to crack propagation at the Si/Al interface. (3) When the de-lithiation stage initiated, a fraction of the amorphous structure transformed back to crystalline and at the end of de-lithiation, localized amorphization at the Si crack faces promoted crack closure due to volume expansion. Bhattacharya and Alpas (2016) Click to enlarge.

In summary, composite electrode materials consisting of uniformly distributed Si embedded in a ductile and inert phase having high fracture toughness could reduce the propensity for electrode fragmentation and improve battery electrode durability.

—Bhattacharya and Alpas (2016)

Resources

• Sandeep Bhattacharya, Ahmet T. Alpas (2016) “Self-healing of cracks formed in Silicon-Aluminum anodes electrochemically cycled at high lithiation rates,” Journal of Power Sources, Volume 328, Pages 300-310 doi: 10.1016/j.jpowsour.2016.07.118