Electrochemical shock : mechanical degradation of ion-intercalation materials

Abstract

The ion-intercalation materials used in high-energy batteries such as lithium-ion undergo large composition changes-which correlate to high storage capacity-but which also induce structural changes and stresses that can cause performance metrics such as power, achievable storage capacity, and life to degrade. "Electrochemical shock"-the electrochemical cycling-induced fracture of materials-contributes to impedance growth and performance degradation in ion-intercalation batteries. Using a combination of micromechanical models and in operando acoustic emission experiments, the mechanisms of electrochemical shock are identified, classified, and modeled in targeted model systems with different composition and microstructure. Three distinct mechanisms of electrochemical shock in ion-intercalation mate- rials are identified: 1) concentration-gradient stresses which arise during fast cycling, 2) two- phase coherency stresses which arise during first-order phase-transformations, and 3) inter-granular compatibility stresses in anisotropic polycrystalline materials. While concentration- gradient stresses develop in proportion to the electrochemical cycling rate, two-phase coherency stresses and intergranular compatibility stresses develop independent of the electro- chemical cycling rate and persist to arbitrarily low rates. For each mechanism, a micromechanical model with a fracture mechanics failure criterion is developed. This fundamental understanding of electrochemical shock leads naturally to microstructure design criteria and materials selection criteria for ion-intercalation materials with improved life and energy storage efficiency. In a given material system, crystal symmetry and phase-behavior determine the active mechanisms. Layered materials, as exemplified by LiCoO₂, are dominated by intergranular compatibility stresses when prepared in polycrystalline form, and two-phase coherency when prepared as single crystal powders. Spinel materials such as LiMn₂O₄, and LiMn₁.₅Ni₀.₅O₄ undergo first-order cubic-to-cubic phase- transformations, and are subject to two-phase coherency stresses even during low-rate electrochemical cycling. This low-rate electrochemical shock is averted in iron-doped material, LiMn₁.₅Ni₀.₄₂Fe₀.₀₈O₄, which has continuous solid solubility and is therefore not subject to two-phase coherency stresses; this enables a wider range of particle sizes and duty cycles to be used without electrochemical shock. While lithium-storage materials are used as model systems, the physical phenomena are common to other ion-intercalation systems, including sodium-, magnesium-, and aluminum-storage compounds.