Investigation of “Na2/3Co2/3Ti1/3O2” as a multi-phase positive electrode material for sodium batteries

Abstract

Co-based layered “Na2/3Co2/3Ti1/3O2” oxide is prepared via a solid-state process and investigated in sodium half-cells. X-ray diffraction reveals the presence of several phases in the pristine material with P2 and P3 structure types as the main phases.The mixture of phases shows a smooth charge/discharge curve and the potential steps previously observed for Na2/3Co1-yTiyO2 (y = 0, 0.05, 0.1) have completely vanished. This phase mixture delivers high charge and discharge capacities of 110.5 and104.5 mAh.g−1, respectively, in the potential range 2–4.2 V as well as a high rate capability. The desodiation/sodiation process of “Na2/3Co2/3Ti1/3O2” is investigated by operando diffraction using synchrotron radiation. The P2 structure-type remains stable in the investigated potential window and dominates the electrochemical activity. Electrochemical impedance spectroscopy was conducted during the first charge/discharge cycle and over cycling. The results show that, from OCV till the end of charge, two semi-circles appear in the Nyquist diagram and merge during the following discharge. The total cell impedance increases drastically during Na-insertion at high sodium content (from 50 Ω at 4 V to 375 Ω at 2 V).