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

• Published in: Energies (Basel) Vol. 15; no. 9; p. 3371
• Main Authors: Pohle, Björn, Gorbunov, Mikhail, Lu, Qiongqiong, Bahrami, Amin, Nielsch, Kornelius, Mikhailova, Daria
• Format: Journal Article
• Language: English
• Published: Basel MDPI AG 05.05.2022
• Subjects: Aluminum; Anodic protection; Batteries; Carbon; Cathodes; Cathodic protection; cation design of Na-cathodes; Cations; Charging; Crystallography; Cycles; Electrochemical analysis; Electrochemistry; Electrodes; Electrolytes; Energy storage; Na-cathode protective strategy; Na-metal protective strategy; Phase transitions; Protective coatings; Reaction mechanisms; Sodium; Sodium-ion batteries; structural transition in layered Na-oxides; Unit cell
• ISSN: 1996-1073; 1996-1073
• DOI: 10.3390/en15093371

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.