Jonas Kaufman, University of California, Santa Barbara
The research community is looking "beyond Li-ion" toward Na- and K-ion battery technologies as low-cost solutions for large-scale energy storage applications. However, these new chemistries present their own unique challenges for developing suitable electrode materials. Within the same type of layered transition-metal oxide intercalation compounds that have been successfully used to cycle Li, inserting Na or K results in two important phenomena: The larger size and stronger electrostatics of these ions can stabilize intricate ion-vacancy orderings and drive structural phase transitions involving stacking sequence changes of the oxide layers. Both effects can be detrimental to battery performance.
In order to better understand the fundamental behavior of layered Na and K intercalation compounds, we have studied several model systems using first-principles techniques. We rely on CASM (a Clusters Approach to Statistical Mechanics), an open-source software package for modeling multi-component crystalline solids with almost arbitrary degrees of freedom. Formation energies of many ion-vacancy orderings within different host structures were calculated using density functional theory. Effective cluster expansion Hamiltonians were fit with a genetic algorithm and then used to predict ground states and determine finite temperature properties through grand canonical Monte Carlo simulations. Our results reveal several families of complex hierarchical orderings that are based on antiphase boundaries. Some of the orderings are accompanied by significant structural distortions of the host layers. The calculated voltage profiles are in good qualitative agreement with experiment. In addition, we have examined generalized-stacking-fault energy surfaces as a way to probe the kinetics of structural phase transitions. These energies also serve as an important parameter for multiscale modeling efforts.
Abstract Author(s): Jonas Kaufman, Anton Van der Ven