Oxygen nonstoichiometry and actual Co valence inNaxCoO2−δ

Abstract

The Na x CoO 2−␦ system-a host lattice for superb thermoelectrics and parent of the watery CoO 2 -layer superconductor-is found to be oxygen nonstoichiometric for x ഛ 0.7, such that with decreasing x, the concentration of oxygen vacancies increases. Accordingly, upon Na deintercalation, the Co valence first increases but does not exceed the value of 3.5. To reveal this, the actual valence of Co was systematically determined by means of wet-chemical redox analysis for a series of Na x CoO 2−␦ samples with the Na content x ranging from 0.78 down to 0.12. The low-Na-content samples were obtained from Na-rich ones ͑0.70ഛ x ഛ 0.78͒ through chimie douce extraction of Na utilizing I 2 , Br 2 , or NO 2 BF 4 as an oxidizing agent. For the lowest-Na-content sample of x = 0.12, the oxygen nonstoichiometry parameter ␦ was 0.23. The ␦-x relation presently established for a wide x range of Na x CoO 2−␦ is indispensable for proper interpretations of physical properties of this system.

Electronic phase diagram of the layered cobalt oxide systemLixCoO2(0.0≤xMotohashi1, Ono2, Sugimoto3 et al. 2009Phys. Rev. B838800View full textAdd to dashboardCite

“…The chemical oxidation of Na x CoO 2 ͑x Ϸ 0.7͒ was previously examined by means of oxidizing reagents including Br 2 and NO 2 BF 4 but the attainable x was limited down to 0.15. 8 For deeper understanding of the electronic structure of Na x CoO 2 , information on this compositional regime is highly desirable. Particularly, CoO 2 , the x = 0 end member is important since it can be considered as a parent phase of Na x CoO 2 .…”

Mechanical spectrum and oxygen vacancies in γ-Na0.66CoO2

“…The formal Co oxidation state is determined by the Na concentration and the charge neutrality in Na x CoO 2−ı where oxygen content is expected perfectly to be 2. To investigate the oxygen nonstoichiometry parameter ı, wet-chemical redox analysis is performed in a series of Na x CoO 2−ı samples with the Na content x ranging from 0.78 down to 0.12 [31]. The marked oxygen nonstoichiometry is observed for the low Na content sample.…”

Structural changes of NaxCoO2 (x=0.74) at high pressures

“…Previous experimental results [1][2][3][4][5][6][7][8][9][10] indicate that both the structural and electronic state of Na x CoO 2 is very rich with the change of the Na content x. There are at least five phases of Na x CoO 2 with different Na content x.…”

“…They have received considerable attentions recently [2][3][4][5][6] not only in the physical aspect of the two-dimensional (2D) nature, but also in their potentials for practical applications, such as Li x CoO 2 has already been used as a cathode material [7] in commercialized lithium-ion rechargeable batteries. The recent interest in the Na counterpart is particularly due to its potential for thermoelectrics [8] and the discovery of the superconducting phase [9] of hydrated Na 0.3 CoO 2 Á 1.3H 2 O.…”