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

Watcharatharapong, Teeraphat and T-Thienprasert, Jiraroj and Barpanda, Prabeer and Ahuja, Rajeev and Chakraborty, Sudip (2017) Mechanistic study of Na-ion diffusion and small polaron formation in Krohnkite Na2Fe(SO4)(2)center dot 2H(2)O based cathode materials. In: JOURNAL OF MATERIALS CHEMISTRY A, 5 (41). pp. 21726-21739.

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Abstract

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.

Item Type:	Journal Article
Publication:	JOURNAL OF MATERIALS CHEMISTRY A
Additional Information:	Copy right for this article belongs to the ROYAL SOC CHEMISTRY, THOMAS GRAHAM HOUSE, SCIENCE PARK, MILTON RD, CAMBRIDGE CB4 0WF, CAMBS, ENGLAND
Department/Centre:	Division of Chemical Sciences > Materials Research Centre
Date Deposited:	17 Nov 2017 05:32
Last Modified:	17 Nov 2017 05:32
URI:	http://eprints.iisc.ac.in/id/eprint/58253

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