Physico-Chemical Features of Undoped and Fe/Cu-Doped Na0.67MnO2-Layered Cathodes for Sodium Batteries

Abstract

Na0.67MnO2 (NMO) stands out among the layered cathode materials used for sodium batteries due to its high-capacity values, low cost, and environmental friendliness. Unfortunately, many drawbacks arise during cycling, but nanostructure tailoring and doping can help to mitigate them. Our aim was to synthesize undoped and Cu- or Fe-doped NMO samples via the sol-gel route, with a different cooling step to room temperature, i.e., in a natural way or via quenching. The formation of a mixture of polymorphs was observed, as well as differences in the external morphology of the powders’ grains. The use of spectroscopic techniques, Mössbauer spectroscopy for the Fe-doped samples and Electron paramagnetic resonance, allowed us to gain insights into the oxidation states of transition metals and to make suggestions about the magnetic ordering, as well as on the possible presence of magnetic impurities. Cyclic voltammetry and galvanostatic cycling results were interpreted on the basis of the spectroscopic data: the introduction of substituents, in general, worsens the capacity values, due to the decrease in the P2 amount and the introduction of structural distortions. The structural stability of the samples in air as a function of time was also analyzed via X-ray diffraction, demonstrating the positive effect of Cu presence.

High-Performance Ag-Doped Na0.67MnO2 Cathode: Operando XRD Study and Full-Cell Performance Analysis with Presodiated Anode

“…The two polymorphs of Na 0.67 MnO 2 , P2 and P2′ (shown in Figure S2), usually appear as mixed phases in a synthesized material, where the amounts of these phases were highly affected by the heating process and uncontrollable Na-ion loss. , The XRD data of NMO are shown in Figure S3 where the inset clearly indicates the presence of both P2 and P2′ phases. Figure a shows the XRD data and Rietveld refinement of NMO-Ag.…”

“…However, in our sample, where these two phases coexist, their cell parameters, both a-and c-axes, show significant differences due to the strain effects between these phases. 13 The interlayer separation between the consecutive Mn−O layers plays a crucial role in the Na insertion/removal process. 31,34 A large interlayer separation provides ample pathways for Na ions to intercalate and deintercalate between the layers.…”

“…The crystallization of these two structures is highly dependent on the synthesis conditions. Due to uncontrollable Na loss, these two phases usually appear as mixed phases in the samples. , P2–P2′ phase transition due to the Jahn–Teller effect is considered to be responsible for the unsatisfactory battery performance of P2-Na 0.67 MnO 2 . The distortion negatively affects the Na + mobility and causes structural collapse during repeated cycles …”