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LiFeSO4F as a Cathode Material for Lithium-Ion Batteries: Synthesis, Structure, and Function
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström. (Structural Chemistry)
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, two recently discovered polymorphs of LiFeSO4F, adopting a tavorite- and triplite-type structure, were investigated as potential candidates for use as cathode materials in Li-ion batteries. The studies aimed at enriching the fundamental understanding of the synthetic preparations, structural properties, and electrochemical functionality of these materials.

By in situ synchrotron X-ray diffraction (XRD), the formation mechanism of the tavorite-type LiFeSO4F was followed starting from two different sets of precursors, FeSO4∙H2O + LiF, and Li2SO4 + FeF2. The results indicated that the formation of LiFeSO4F is possible only through the structurally related FeSO4∙H2O, in line with the generally recognized topotactic reaction mechanism. Moreover, an in-house solvothermal preparation of this polymorph was optimized with the combined use of XRD and Mössbauer spectroscopy (MS) to render phase pure and well-ordered samples. Additionally, the triplite-type LiFeSO4F was prepared using a facile high-energy ball milling procedure.

The electrochemical performance of as-prepared tavorite LiFeSO4F was found to be severely restricted due to residual traces of the reaction medium (tetraethylene glycol (TEG)) on the surface of the synthesized particles. A significantly enhanced performance could be achieved by removing the TEG residues by thorough washing, and a subsequent application of an electronically conducting surface coating of p-doped PEDOT. The conducting polymer layer assisted the formation of a percolating network for efficient electron transport throughout the electrode, resulting in optimal redox behavior with low polarization and high capacity. In the preparation of cast electrodes suitable for use in commercial cells, reducing the electrode porosity was found to be a key parameter to obtain high-quality electrochemical performance. The triplite-type LiFeSO4F showed similar improvements upon PEDOT coating as the tavorite-type polymorph, but with lower capacity and less stable long-term cycling due to intrinsically sluggish kinetics and unfavorable particle morphology.

Finally, the Li+-insertion/extraction process in tavorite LiFeSO4F was investigated. By thorough ex situ characterization of chemically and electrochemically prepared LixFeSO4F compositions (0≤x≤1), the formation of an intermediate phase, Li1/2FeSO4F, was identified for the first time. These findings helped redefine the (de)lithiation mechanism which occurs through two subsequent biphasic reactions, in contrast to a previously established single biphasic process.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2015. , 79 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1291
Keyword [en]
Li-ion battery, cathode, LiFeSO4F, tavorite, triplite, synthesis, performance, structure, coating, PEDOT, XRD, Mössbauer spectrocopy, SEM, TEM, electrochemistry
National Category
Materials Chemistry
Research subject
Chemistry with specialization in Materials Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-262715ISBN: 978-91-554-9344-8 (print)OAI: oai:DiVA.org:uu-262715DiVA: diva2:855021
Public defence
2015-11-05, Häggsalen, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2015-10-14 Created: 2015-09-18 Last updated: 2015-10-27
List of papers
1. Formation of Tavorite-Type LiFeSO4F Followed by In Situ X-ray Diffraction
Open this publication in new window or tab >>Formation of Tavorite-Type LiFeSO4F Followed by In Situ X-ray Diffraction
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2015 (English)In: Journal of Power Sources, ISSN 0378-7753, E-ISSN 1873-2755, Vol. 298, 363-368 p.Article in journal (Refereed) Published
Abstract [en]

The tavorite-type polymorph of LiFeSO4F has recently attracted substantial attention as a positive elec- trode material for lithium ion batteries. The synthesis of this material is generally considered to rely on a topotactic exchange of water (H2O) for lithium (Li) and fluorine (F) within the structurally similar hy- drated iron sulfate precursor (FeSO4·H2O) when reacted with lithium fluoride (LiF). However, there have also been discussions in the literature regarding the possibility of a non-topotactic reaction mechanism between lithium sulfate (Li2SO4) and iron fluoride (FeF2) in tetraethylene glycol (TEG) as reaction medium. In this work, we use in situ X-ray diffraction to continuously follow the formation of LiFeSO4F from the two suggested precursor mixtures in a setup aimed to mimic the conditions of a solvothermal autoclave synthesis. It is demonstrated that LiFeSO4F is formed directly from FeSO4·H2O and LiF, in agreement with the proposed topotactic mechanism. The Li2SO4 and FeF2 precursors, on the other hand, are shown to rapidly transform into FeSO4·H2O and LiF with the water originating from the highly hygroscopic TEG before a subsequent formation of LiFeSO4F is initiated. The results highlight the importance of the FeSO4·H2O precursor in obtaining the tavorite-type LiFeSO4F, as it is observed in both reaction routes.

National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-243324 (URN)10.1016/j.jpowsour.2015.08.062 (DOI)000362146800044 ()
Funder
VINNOVA, P37446-1Swedish Energy Agency, 30769-2Swedish Research Council, C0468101StandUp
Available from: 2015-02-08 Created: 2015-02-08 Last updated: 2017-12-04Bibliographically approved
2. A Mössbauer spectroscopy study of polyol synthesized tavorite LiFeSO4F.
Open this publication in new window or tab >>A Mössbauer spectroscopy study of polyol synthesized tavorite LiFeSO4F.
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2014 (English)In: Hyperfine Interactions, ISSN 0304-3843, E-ISSN 1572-9540, ISSN 0304-3843, Vol. 226, no 1-3, 229-236 p.Article in journal (Refereed) Published
Abstract [en]

The tavorite polymorph of LiFeSO4F has attracted considerable attention as a cathode material for lithium ion batteries due to interesting structural and electrochemical characteristics. For the analysis of such iron-based electrode materials, Mössbauer spectroscopy has become an important and highly useful tool. In this work, we perform a detailed Mössbauer study of pristine tavoriteLiFeSO4F prepared by an optimized synthesis in tetraethylene glycol as reaction media. In contrast to many reported results, we demonstrate the use of an asymmetric fitting model for the inner doublet of the spectrum, which is coupled to the structural properties of the compound. Moreover, we discuss a new approach of ascribing the Fe2 + -doublets to the two distinct crystallographic iron sites of tavorite LiFeSO4F by comparing the Mössbauer signal intensities with the expected f-factors for the corresponding iron atom.

Keyword
lithium ion battery, tavorite LiFeSO4F, Mössbauer spectroscopy
National Category
Materials Chemistry
Research subject
Physics with specialization in Nuclear Physics; Chemistry with specialization in Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-241039 (URN)10.1007/s10751-013-0935-1 (DOI)
Conference
Proceedings of the 32nd International Conference on the Applications of the Mössbauer Effect (ICAME 2013) held in Opatija, Croatia, 1–6 September 2013.
Available from: 2015-01-08 Created: 2015-01-08 Last updated: 2017-12-05
3. Understanding and Controlling the Surface Chemistry of LiFeSO4F for an Enhanced Cathode Functionality
Open this publication in new window or tab >>Understanding and Controlling the Surface Chemistry of LiFeSO4F for an Enhanced Cathode Functionality
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2013 (English)In: Chemistry of Materials, ISSN 0897-4756, E-ISSN 1520-5002, Vol. 25, no 15, 3020-3029 p.Article in journal (Refereed) Published
Abstract [en]

The tavorite polymorph of LiFeSO4F has recently attracted a lot of interest as a cathode material for lithium ion batteries stimulated by its competitive specific capacity, high potential for the Fe2+/Fe3+ redox couple, and low-temperature synthesis. However, the synthesis routes explored to date have resulted in notably varied electrochemical performance. This inconsistency is difficult to understand given the excellent purity, crystallinity, and similar morphologies achieved via all known methods. In this work, we examine the role of the interfacial chemistry on the electrochemical functionality of LiFeSO4F. We demonstrate that particularly poor electrochemical performance may be obtained for pristine materials synthesized in tetraethylene glycol (TEG), which represents one of the most economically viable production methods. By careful surface characterization, we show that this restricted performance can be largely attributed to residual traces of TEG remaining on the surface of pristine materials, inhibiting the electrochemical reactions. Moreover, we show that optimized cycling performance of LiFeSO4F can be achieved by removing the unwanted residues and applying a conducting polymer coating, which increases the electronic contact area between the electrode components and creates a highly percolating network for efficient electron transport throughout the composite material. This coating is produced using a simple and scalable method designed to intrinsically favor the functionality of the final product.

Keyword
battery, fluorosulfate, polymer coating, powder X-ray diffraction, Mossbauer spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy
National Category
Engineering and Technology Natural Sciences
Identifiers
urn:nbn:se:uu:diva-207530 (URN)10.1021/cm401063s (DOI)000323193000016 ()
Available from: 2013-09-16 Created: 2013-09-16 Last updated: 2017-12-06
4. Insight Into the Electrochemcal Performance of PEDOT-coated Tavorite LiFeSO4F for Li-ion Batteries
Open this publication in new window or tab >>Insight Into the Electrochemcal Performance of PEDOT-coated Tavorite LiFeSO4F for Li-ion Batteries
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(English)Manuscript (preprint) (Other academic)
National Category
Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-262676 (URN)
Available from: 2015-09-18 Created: 2015-09-18 Last updated: 2015-10-27
5. Investigating the Electrochemical Performance of PEDOT-coated Triplite-type LiFeSO4F Cathode Material
Open this publication in new window or tab >>Investigating the Electrochemical Performance of PEDOT-coated Triplite-type LiFeSO4F Cathode Material
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(English)Manuscript (preprint) (Other academic)
National Category
Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-262677 (URN)
Available from: 2015-09-18 Created: 2015-09-18 Last updated: 2016-03-22
6. Identification of an Intermediate Phase, Li1/2FeSO4F, Formed during Electrochemical Cycling of Tavorite LiFeSO4F
Open this publication in new window or tab >>Identification of an Intermediate Phase, Li1/2FeSO4F, Formed during Electrochemical Cycling of Tavorite LiFeSO4F
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2014 (English)In: Chemistry of Materials, ISSN 0897-4756, E-ISSN 1520-5002, Vol. 26, no 15, 4620-4628 p.Article in journal (Refereed) Published
Abstract [en]

Many compounds adopting the tavorite-type crystal structure have attracted considerable attention as cathode materials for lithium ion batteries due to the favorable structural characteristics, facilitating promising electrochemical performance. Recent reports have highlighted the complex mechanism of lithium insertion/extraction in some of these compounds, such as the stabilization of intermediate phases in the LiFeSO4OH and LiVPO4F systems. In the case of tavorite LiFeSO4F, reported density functional theory (DFT) calculations have suggested the possibility of a similar behavior, but thus far, no experimental verification of such a process has, to the best of our knowledge, been successfully demonstrated. In this work, we investigate the structural evolution of LiFeSO4F upon extraction/insertion of lithium ions from/into the host framework. By thorough ex situ characterizations of chemically and electrochemically prepared LixFeSO4F-samples (0 ≤ x ≤ 1), we demonstrate the stabilization of an intermediate phase, Li1/2FeSO4F, for which one possible structural model is proposed. However, results indicating charge ordering on the iron-sites, suggesting the formation of a super structure with a larger unit cell, are also highlighted. Moreover, the degree of formation of Li1/2FeSO4F is shown to be highly dependent on the rate of lithium extraction as a result of an exceptionally small potential separation (similar to 15 mV during charging) of the two subsequently occurring biphasic processes, LiFeSO4F/Li1/2FeSO4F and Li1/2FeSO4F/FeSO4F. Finally, the intermediate phase is shown to be formed both on charge and discharge during battery cycling, even though an apparent asymmetrical electrochemical trace suggests the contrary.

National Category
Physical Sciences Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-232001 (URN)10.1021/cm502104q (DOI)000340346300038 ()
Available from: 2014-09-15 Created: 2014-09-12 Last updated: 2017-12-05

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