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Issue 41, 2017

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

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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 fast-ion diffusion and high reversibility. Owing to the structural complexity, Na+ diffusion was assumed to occur along a convoluted channel along the 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 the Na+ diffusion mechanism in this material. The electronic structure and charge transfer are also envisaged to reveal evidence of Fe2+/3+ redox reaction and a vital role of structural H2O. Based on formation energies of this material with varied Na concentration, a calculated voltage profile is determined to show two voltage plateaus at 4.81 and 3.51 V, corresponding to experimental results. Nudged elastic band calculation reveals that Na+ diffusion is primarily occuring in the [01[1 with combining macron]] 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.

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

Supplementary files

Article information


Submitted
23 May 2017
Accepted
12 Sep 2017
First published
13 Sep 2017

J. Mater. Chem. A, 2017,5, 21726-21739
Article type
Paper

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, 5, 21726 DOI: 10.1039/C7TA04508E

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