Electrochemical Shock in Ion-Intercalation Materials with Limited Solid-Solubility

We present a particle-level model for electrochemical shock in phase-transforming electrode particles and apply the model to spinel LiXMn2O4. Two-phase-coherency stresses are the dominant electrochemical shock mechanism in this material, due to the lithium miscibility gap 0.25 <= X <= 0.5, and can cause particle fracture even during slow charging. This demonstrates that C-Rate-independent electrochemical shock is possible in cubic compounds, where the compositional strains are isotropic. Using LiXMn2O4 as a model system, we study the competition between C-Rate-dependent concentration-gradient stresses and C-Rate-independent two-phase-coherency stresses, and find that at practical C-Rates and particle sizes, phase-transformation induced coherency-stresses are the dominant electrochemical shock mechanism. This model predicts a critical particle size for LiXMn2O4 on the order of one micron-with a weak dependence on C-Rate-below which coherency-stress fracture is avoided in LiXMn2O4; below this size, only C-Rate-dependent concentration-gradient stresses will contribute to electrochemical shock. As many technologically important lithium-storage compounds have complex phase-behavior with miscibility gaps, two-phase-coherency stresses may be the most common electrochemical shock mechanism. Crystal chemical engineering of lithium-storage compounds to shrink miscibility-gaps and/or to reduce the misfit strains between coexisting phases is a viable strategy to avoid electrochemical shock. While the present work focuses on lithium-intercalation compounds, it is applicable to all ion-intercalation compounds, including sodium- and magnesium-storage materials. (C) 2013 The Electrochemical Society.