(Invited) Mechano-Electro-Chemical Coupling in Lithium Intercalation Compounds

Mechano-Electro-Chemical (MEC) Coupling in intercalation compounds can be considered in multiple ways.  Large stresses caused by electrochemically induced stoichiometry changes can lead to local stress and mechanical degradation.  On the other hand, MEC coupling may also provide new opportunities to characterize materials by measuring the stress/strain generated by the stoichiometry change.

One typical challenge in modeling the MEC coupling is the lack of proper material properties as a function of the state of charge (SOC). To address this challenge, we developed “atomically informed” battery mechanics models by integrating density functional theory (DFT) predicted mechanical properties into continuum models to predict diffusion induced deformation and stress evolution and make direct comparisons with experiments. It was found that lithiation induced stiffening occurs in layered compounds, however the averaged Young’s moduli change very minimally at a function of SOC in the spinel and olivine structures. Depends on the tendency of Li to segregate to or deplete from the fractured surface, the work of decohesion of electrodes can decrease or increase with SOC, and crack instability may also occur due to phase transitions within the decohering region.

Measuring the strain due to stoichiometry change can be further complicated by the coupling of different vacancy species. For example, in Li-excess layered compounds (xLi₂MnO₃∙(1-x)LiMO₂, where M=Ni, Co and/or Mn), it is widely agreed that oxygen vacancies can be introduced to the Li₂MnO₃ phase during the initial charging cycles. With DFT and Ab initio molecular dynamics calculations, we demonstrate that correlations exist between oxygen vacancy and the formation and migration of Li vacancies. As Li vacancies tend to form and cluster around oxygen vacancies, the oxygen vacancies hinder the diffusion of the Li ions. On the other hand, the nonstoichiometry strains are correlated between the two species, making the MEC coupling analysis more complex in these intercalation compounds.