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Mechanistic study of Na-ion diffusion and small polaron formation in Krohnkite Na2Fe(SO4)(2)center dot 2H(2)O based cathode materials
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Uppsala University, Sweden.
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2017 (English)In: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 5, no 41, p. 21726-21739Article in journal (Refereed) Published
Abstract [en]

Krohnkite-type Na2Fe(SO4)(2)center dot 2H(2)O 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) over bar] 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.

Place, publisher, year, edition, pages
ROYAL SOC CHEMISTRY , 2017. Vol. 5, no 41, p. 21726-21739
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Materials Engineering
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URN: urn:nbn:se:kth:diva-217410DOI: 10.1039/c7ta04508eISI: 000413734800014Scopus ID: 2-s2.0-85032358182OAI: oai:DiVA.org:kth-217410DiVA, id: diva2:1159000
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QC 20171121

Available from: 2017-11-21 Created: 2017-11-21 Last updated: 2017-11-21Bibliographically approved

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Ahuja, Rajeev

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