Synthesis and Stoichiometry of Different Layered Sodium Cobalt Oxides

Abstract

Layered Na–metal oxides can form in different crystal structures, each with different electrochemical behavior. As a prototype system to better understand how each phase can be formed, we present the conditions under which different layered phases of Na x CoO2 can be stabilized in solid-state synthesis. Using a novel combination of ex situ XRD on as-synthesized samples, with in situ XRD to monitor the relation between Na content and lattice parameters, we are able to construct a phase diagram of Na x CoO2 between 450 to 750 °C in air and for Na:Co sample ratios ranging from 0.60 to 1.05. Four single phase domains of O3, O′3, P′3, and P2 are revealed based on the XRD analysis. In contrast to previous reports it is found that pure O3, O′3 and P′3 phase can only form at a fixed stoichiometry of x = 1.00, 0.83, and 0.67, respectively, while the P2 phase forms in a slightly larger composition range from 0.68 to 0.76. Galvanostatic charging of O3–Na1.00CoO2 shows several flat and sloping regions on the voltage profile, which follows the sequence of O3–O′3–P′3–P3–P′3, with increasing interslab distances. Our results indicate that the electrochemically important P2 structure is likely stabilized by entropy.

Dual Stabilization and Sacrificial Effect of Na2CO3 for Increasing Capacities of Na-Ion Cells Based on P2-NaxMO2 Electrodes

“…On that basis, we can understand why excess of Na favours the formation of the P2 phase which additionally preserves it to room temperature by slow cooling in contrast to previous reports. 28 At this stage, it is also worth mentioning that the P2 phases formed under such conditions are stable even after washing the excess sodium carbonate with water and re-annealing at 800 °C for 12 hours ( Figure S7 in SI). Altogether, excess sodium in the reaction mixture acts as a promoter for the P3-P2 phase transformation.…”

Review—Manganese-Based P2-Type Transition Metal Oxides as Sodium-Ion Battery Cathode Materials

“…On the other hand, an experimental study by Lei et al explored the Na x CoO 2 synthesis phase diagram. Na x MO 2 compounds are typically prepared via solid-state synthesis, 8,57 or a solution-state co-precipitation method, 9,12 with calcination temperatures ranging from 500 to 1000…”

Von Lithium‐ zu Natriumionenbatterien: Vorteile, Herausforderungen und Überraschendes

“…Bei Na-basierten Schichtoxiden vom O3-Typ,wie NaCoO 2 kann nur ein geringer Na-Anteil aus der Struktur ausgelagert werden bevor die Struktur instabil wird, was zu einer relativ geringen Kapazitätv on etwa 115 mAh g À1 führt. [69] Die Na-basierten Schichtoxide vom P2-Typ,w ie Na 2/3 MnO 2 ,k çnnen ihre Struktur bis zur Extraktion von 0.4 Na aufrechterhalten, jenseits davon kann sich die P2-Struktur in die OP4-Phase umwandeln. [70] P2-Na 2/3 Ni 1/3 Mn 2/3 O 2 wandelt sich bei Extraktion von 0.34 Na in die O3-Phase um.…”