High-Capacity Na-Excess Positive Electrode Materials; Na3NbO4-NaMnO2 Binary System

Abstract

The research interest for rechargeable sodium batteries is rapidly growing, especially for the large scale energy storage. Recently, we have reported a new Li-excess positive electrode material, Li3NbO4 – LiMnO2,[1] which effectively utilizes solid-state redox reaction of oxide ions, instead of classical redox of transition metals. The use of Na-excess materials and anion redox is important strategy to further increase the energy density of rechargeable sodium batteries.[2] In this study, Na3NbO4 is targeted as a model material and the possibility of new high-capacity electrode materials with the anion redox is examined. Our trials to synthesize Na3NbO4-NaMnO2 binary system by a conventional solid-state method were, however, failed, and therefore the materials were prepared via a non-equilibrium route.NaMnO2 was prepared by solid-state reaction from Na2CO3 (99.5 %, Wako Pure Chemical Industries, Ltd) and Mn2O3 and Na3NbO4 was prepared from Na2CO3 and Nb2O5 (99.9 %, Wako Pure Chemical Industries, Ltd). Both oxides were used as precursors. A mixture of Na3NbO4 and NaMnO2 was ball-milled using a ZrO2 pot and balls. Figure 1a shows X-ray diffraction patterns of the samples of x = 0.43 in x Na3NbO4 – (1–x) NaMnO2 binary system as a function of milling time. The chemical formula of the sample is reformulated as Na1.3Nb0.3Mn0.4O2. The mixture of Na3NbO4 and NaMnO2changes into cation-disordered rocksalt phase after the mechanical milling for 24 h. SEM images before and after ball-milling (24 h) are also compared in Figure 1b. Particle size are decreased to nano scale with spherical morphology. Moreover, thus prepared sample delivers large reversible capacity which cannot be explained on the basis of redox reaction of the transition metal ions.From these results, we will discuss the possibility of Na-excess positive electrode materials with anion redox for rechargeable Na batteries.