Robustness of amorphous silicon during the initial lithiation/delithiation cycle

Recent research on the electrochemical lithiation of amorphous silicon nanoparticles shows that amorphous silicon is more fracture resistant than crystalline silicon during lithiation. Nanoparticles of amorphous silicon can be lithiated and delithiated without any fracture at all. To fully exploit the potential of using amorphous silicon as electrodes for lithium ion batteries it is important to determine if larger, micron-sized, amorphous silicon structures can be lithiated and delithiated without fracture. Here we study the morphologies of initially amorphous silicon micropillars (∼2.3 μm tall) both before and after electrochemical lithiation and delithiation. No internal or external cohesive cracking is detected in lithiated pillars for any of the pillar sizes studied. Delithiated pillars exhibit some delamination at the interface between the pillar and the underlying nickel substrate. For larger diameter pillars, the initiated interfacial crack is driven upward into the delithiated pillar as the crack propagates radially inward. However, no cohesive fracture unrelated to interfacial cracking is seen in even the largest delithiated pillars. Finite element modeling provides support for the observation that the cohesive fracture resistance of amorphous silicon micropillars is representative of the fracture resistance of amorphous silicon microparticles of comparable dimensions.