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Mechanistic study of Na-ion diffusion and small polaron formation in Kröhnkite Na2Fe(SO4)2·2H2O based cathode materials
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
Kasetsart Univ, Fac Sci, Dept Phys, Bangkok 10900, Thailand..
Indian Inst Sci, Mat Res Ctr, Faraday Mat Lab, Bangalore 560012, Karnataka, India..
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Royal Inst Technol KTH, Dept Mat & Engn, Appl Mat Phys, S-10044 Stockholm, Sweden..ORCID iD: 0000-0003-1231-9994
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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]

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] 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
2017. Vol. 5, no 41, p. 21726-21739
National Category
Materials Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-340686DOI: 10.1039/c7ta04508eISI: 000413734800014OAI: oai:DiVA.org:uu-340686DiVA, id: diva2:1179833
Funder
Swedish Research CouncilCarl Tryggers foundation StandUp
Note

Title in WoS: Mechanistic study of Na-ion diffusion and small polaron formation in Krohnkite Na2Fe(SO4)(2)center dot 2H(2)O based cathode materials

Available from: 2018-02-02 Created: 2018-02-02 Last updated: 2018-02-02Bibliographically approved

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Watcharatharapong, TeeraphatAhuja, RajeevChakraborty, Sudip

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