Ordering and Structural Transformations in Layered KxCrO2 for K-Ion Batteries

The success of K-ion battery technology will rely on the development of robust cathode materials that can incorporate and shuttle large amounts of K reversibly. Recent experimental work has demonstrated the viability of layered KCrO2 as a cathode material for K-ion batteries; however, some fundamental details of structural phase transitions and K ordering during cycling remain unknown. We report on a first-principles thermodynamic investigation of layered KxCrO2 (0 ≤ x ≤ 1) in the O3 and P3 host structures. We predict that P3 is preferred at intermediate x, with the stable K orderings belonging to staircases of phases that contain antiphase boundaries between ordered regions. Varying densities of these boundaries allow for smooth changes in composition. At high x, we predict the stability of "M" phases containing a mixture of octahedral and prismatic K coordination within each layer, which is accommodated by undulations of the oxide host. Our calculated voltage curve and analysis of structural evolution indicate that the predicted phase stability, including the formation of the M phases, is mostly compatible with experimental observations.