Voltage hysteresis loop as a fingerprint of slow kinetics Co2+-to-Co3+ transition in layered NaxCox/2Ti1-x/2O2 cathodes for sodium batteries

Sodium transition metal oxides are one of the most promising cathode materials for future sodium ion batteries. Chemical flexibility of layered Na-oxides including cobalt enables its partial substitution by other redox-active or non-active metals, often leading to structural stabilization. Sharing the same structural positions with other transition metals in layered oxides, Co can be double- or triple-charged, and as Co3+ can adopt a low-spin (LS), intermediate-spin (IS), high-spin (HS) state, or a combination of them. Using Ti4+ in the structure together with Co2+ results in a reduced number of phase transformations compared to Ti-free compositions. However, a large potential hysteresis of about 1.5-2.5 V between battery charge and discharge is observed, pointing a first-order cooperative phase transition. Based on several examples, we found that Na extraction from NaxCox/2Ti1-x/2O2 materials with high-spin HS-Co2+, crystallizing in the P2 or O3 structure, mostly results in valence and spin-state transition of Co, leading to the formation of a second phase with a low-spin LS-Co3+, and a much smaller unit cell volume. We elucidated a kinetic origin of the potential hysteresis, which can be minimized by increasing temperature or reduction of the current density during battery cycling with P2- and O3-Na0.67Co0.33Ti0.67O2 materials. The slow kinetics of the structural phase transition, especially upon Na-insertion, hampers the application of classical methods of electrochemical thermodynamics, such as determining the entropic potential dE/dT. We showed that the entropic potential depends only on the Na-content in NaxCo0.33Ti0.67O2 during battery charge or discharge, what additionally confirms a kinetic nature of the potential hysteresis.