“Na Redistribution” Induced By K Intercalation during Na/K Ion Exchange in a Layered Oxide Cathode

Abstract

An ion exchange reaction that can stabilize potassium transition metal oxides was proposed as a new approach to develop cathode materials for K-ion batteries (KIBs).1-4 Such ion exchange method indeed has frequently used for the development of novel Li-layered oxides to attain structural features of Na layered oxides.5-7 Although the ion exchange reaction has the potential to discover novel layered cathode materials by accessing the metastable states that cannot be achieved by conventional high-temperature solid-state reaction, the solid-state ion-exchange mechanism and the resulting phase evolution are barely understood.In this study, we systematically investigate electrochemical Na+/K+ ion exchange in layered oxides. 8 Here, O3-type Na3Ni2SbO cathode is used as a model system.8 We found intriguing phase evolutions according to the relative Na/K content upon K intercalation by in-situ X-ray diffraction study. First, we observe that K-rich and Na-rich phases coexist during charging and discharging. Second, Na-rich phase goes through phase transformation upon K intercalation. Strong electrostatic repulsion between Na and K ions that have radically different bond lengths to coordinating oxygen ions can result in this complex phase evolutions. We uncovered that the intercalating K ions pushes away Na ions remaining in the desodiated Na x Ni2SbO6 phase, thereby separating Na-rich and K-rich phases in a particle. Interestingly, this phenomenon creates complex, three-phase equilibrium during the desodiation and potassiation processes in Na x K y Ni2SbO6: phase equilibrium between one K-rich and two Na-rich phases or two K-rich and one Na-rich phase is observed, as dictated by the Gibbs phase rule. Our computational study further demonstrated that this phase separation originates from the large lattice mismatch along the c-axis between the K-rich and Na-rich phases. Our observations and interpretations demonstrate that “ion-exchanged” systems may be more complex to interpret than the previously understood. Our analysis should be applicable to other ion-exchange systems where the exchanged ions are not well miscible in the host structure.