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Mechanistic study of Na-ion diffusion and small polaron formation in Kröhnkite Na2Fe(SO4)2·2H2O based cathode materials

Abstract

Kröhnkite-type Na2Fe(SO4)2·2H2O mineral is a sustainable and promising polyanionic cathode that has been experimentally found to offer a high redox potential (3.25 V vs. Na/Na+) along with a fast-ion diffusion and high reversibility. Owing to the structural complexity, Na+ diffusion was assumed to occur along a convoluted channel in b-axis. However, theoretical work related to this material still appears missing to support that statement. In this work, DFT+U calculations have been performed with the primary aim to unveil Na+ diffusion mechanism in this material. The electronic structure and charge transfer are also envisaged to describe an evidence of Fe2+/3+ redox reaction and a vital role of structural H2O. Based on formation energies of this material with varied Na concentration, calculated voltage profile is determined to show two voltage plateaus at 4.81 and 3.51 V corresponding to experiments. Nudged elastic band calculation reveals that Na+ diffusion is primarily prevailed in direction with a moderate ionic mobility due to the structural distortion induced during migration, suggesting the possibility of defect-assisted diffusion. Intriguingly, the formation of small hole polarons is first observed and could play a key role in the electronic conduction of this material.

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Publication details

The article was received on 23 May 2017, accepted on 12 Sep 2017 and first published on 13 Sep 2017


Article type: Paper
DOI: 10.1039/C7TA04508E
Citation: J. Mater. Chem. A, 2017, Accepted Manuscript
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    Mechanistic study of Na-ion diffusion and small polaron formation in Kröhnkite Na2Fe(SO4)2·2H2O based cathode materials

    T. Watcharatharapong, J. T. Thienprasert, P. Barpanda, R. Ahuja and S. Chakraborty, J. Mater. Chem. A, 2017, Accepted Manuscript , DOI: 10.1039/C7TA04508E

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