Operando Studies on the NaNi0.5Ti0.5O2 Cathode for Na-Ion Batteries: Elucidating Titanium as a Structure Stabilizer.

O3-type layered NaNi0.5Ti0.5O2, which has been reported previously as a promising cathode material for Na-ion batteries, has been characterized using comprehensive operando techniques combined with electrochemical and magnetization measurements. Operando Synchrotron diffraction revealed a reversible O3-P3 transformation during charge and discharge without any intermediate phases, which stands in contrast to NaNiO2 and NaNi0.5Mn0.5O2. Operando X-ray absorption studies showed that the electrochemical process in the potential window of 1.5-4.2 V vs Na+/Na is sustained exclusively by Ni oxidation and reduction while Ti remains inactive. These findings are further supported by ex situ magnetization measurements, yielding a lower paramagnetic moment in the charged state in agreement with Ni oxidation. On the basis of these insights, we elaborate on the beneficial stabilizing effect of Ti. However, a strong C-rate dependence for NaNi0.5Ti0.5O2 and NaNi0.5Mn0.5O2 during cycling known from the literature points at a rather high influence of the original structure stacking and the associated Na migration paths.