  Long-term durability is a major obstacle limiting the widespread use of
lithium ion batteries (LIBs) in heavy-duty applications and others demanding
extended lifetime. As one of the root causes of degradation and failure of
battery performance, the electrode failure mechanisms are still unknown. Here,
we reveal the fundamental fracture mechanisms of single-crystal silicon
electrodes over extended lithiation/delithiation cycles, using electrochemical
testing, microstructure characterization, fracture mechanics, and finite
element analysis. Anisotropic lithium invasion causes crack initiation
perpendicular to the electrode surface, followed by growth through the
electrode thickness. The low fracture energy of the lithiated/unlithiated
silicon interface provides a weak microstructural path for crack deflection,
accounting for the crack patterns and delamination observed after repeated
cycling. Based on this physical understanding, we demonstrate how electrolyte
additives can heal electrode cracks and provide strategies to enhance the
fracture resistance in future LIBs from surface chemical, electrochemical, and
material science perspectives.
