Irreversible Made Reversible: Increasing the Electrochemical Capacity by Understanding the Structural Transformations of Na xCo0.5Ti0.5O2

Structural and Electrochemical Properties of Layered P2-Na0.8Co0.8Ti0.2O2 Cathode in Sodium-Ion Batteries

Layered Na0.8Co0.8Ti0.2O2 oxide crystallizes in the β-RbScO2 structure type (P2 modification) with Co(III) and Ti(IV) cations sharing the same crystallographic site in the metal-oxygen layers. It was synthesized as a single-phase material and characterized as a cathode in Na- and Na-ion batteries. A reversible capacity of about 110 mA h g−1 was obtained during cycling between 4.2 and 1.8 V vs. Na+/Na with a 0.1 C current density. This potential window corresponds to minor structural changes during (de)sodiation, evaluated from operando XRD analysis. This finding is in contrast to Ti-free NaxCoO2 materials showing a multi-step reaction mechanism, thus identifying Ti as a structure stabilizer, similar to other layered O3- and P2-NaxCo1−yTiyO2 oxides. However, charging the battery with the Na0.8Co0.8Ti0.2O2 cathode above 4.2 V results in the reversible formation of a O2-phase, while discharging below 1.5 V leads to the appearance of a second P2-layered phase with a larger unit cell, which disappears completely during subsequent battery charge. Extension of the potential window to higher or lower potentials beyond the 4.2–1.8 V range leads to a faster deterioration of the electrochemical performance. After 100 charging-discharging cycles between 4.2 and 1.8 V, the battery showed a capacity loss of about 20% in a conventional carbonate-based electrolyte. In order to improve the cycling stability, different approaches including protective coatings or layers of the cathodic and anodic surface were applied and compared with each other.

Comparison of Layered Li(Li0.2Rh0.8)O2 and LiRhO2 upon Li Removal: Stabilizing Effect of Li Substitution

Phase transformations upon delithiation in layered oxides with the NaCrS₂ structure type are widely studied for numerous combinations of 3d transition metals because of the application of LiCoO₂ and its derivatives as cathode materials in rechargeable Li-ion batteries. However, complete replacement of 3d by 4d transition metals still yields phenomena never seen in compounds containing 3d metals only. In the present work, the structural evolution of Li-rich O3-Li(Li_0.2Rh_0.8)O₂, having a mixed occupancy of 20% Li and 80% Rh in the metal-O slabs, was studied during electrochemical Li removal and insertion and compared with the isostructural stoichiometric LiRhO₂. The latter compound undergoes a transformation from the layered NaCrS₂ to the tunnel-like rutile-ramsdellite intergrowth structure of the γ-MnO₂ type. Partial replacement of Rh by Li, in contrast, completely prevents this transition, resulting in a reversible cell expansion and shrinkage within the layered structure upon (de)lithiation. Moreover, no anomalously short Rh-O and O-O distances were observed in Li_x≈0(Li_0.2Rh_0.8)O₂ with the Rh^4.75+ intermediate valence state at 4.8 V, in contrast to Li_x≈0RhO₂ with Rh⁴⁺ at 4.2 V, as confirmed by operando synchrotron X-ray diffraction and extended X-ray absorption fine structure studies. We believe that the difference in the Li-O and Rh-O covalency is responsible for the observed structural stabilization. The longer and more ionic Li-O bonds in the (Li,Rh)O₂ layers impede the shortening of O-O distances needed for transformation to the γ-MnO₂ type because of a higher negative charge on O anions connected to Li cations and the stronger electrostatic repulsion between them.

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

O3-type layered NaNi_0.5Ti_0.5O₂, 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 NaNi_0.5Mn_0.5O₂. 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 NaNi_0.5Ti_0.5O₂ and NaNi_0.5Mn_0.5O₂ during cycling known from the literature points at a rather high influence of the original structure stacking and the associated Na migration paths.

Suppression of Monoclinic Phase Transitions of O3-Type Cathodes Based on Electronic Delocalization for Na-Ion Batteries

As high capacity cathodes, O3-type Na-based oxides always suffer from a series of monoclinic transitions upon sodiation/desodiation, mainly caused by Na⁺/vacancy ordering and Jahn-Teller (J-T) distortion, leading to rapid structural degradation and serious performance fading. Herein, a simple modulation strategy is proposed to address this issue based on refrainment of electron localization in expectation to alleviate the charge ordering and change of electronic structure, which always lead to Na⁺/vacancy ordering and J-T distortion, respectively. According to density functional theory calculations, Fe³⁺ with slightly larger radius is introduced into NaNi_0.5Mn_0.5O₂ with the intention of enlarging transition metal layers and facilitating electronic delocalization. The obtained NaFe_0.3Ni_0.35Mn_0.35O₂ exhibits a reversible phase transition of O3_hex-P3_hex without any monoclinic transitions in striking contrast with the complicated phase transitions (O3_hex-O'3_mon-P3_hex-P'3_mon-P3'_hex) of NaNi_0.5Mn_0.5O₂, thus excellently improving the capacity retention with a high rate kinetic. In addition, the strategy is also effective to enhance the air stability, proved by direct observation of atomic-scale ABF-STEM for the first time.