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Investigating the Electrochemical Performance of PEDOT-coated Triplite-type LiFeSO4F Cathode Material
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström.
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(English)Manuscript (preprint) (Other academic)
National Category
Materials Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-262677OAI: oai:DiVA.org:uu-262677DiVA: diva2:855002
Available from: 2015-09-18 Created: 2015-09-18 Last updated: 2016-03-22
In thesis
1. LiFeSO4F as a Cathode Material for Lithium-Ion Batteries: Synthesis, Structure, and Function
Open this publication in new window or tab >>LiFeSO4F as a Cathode Material for Lithium-Ion Batteries: Synthesis, Structure, and Function
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
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:nbn:se:uu:diva-262715 (URN)978-91-554-9344-8 (ISBN)
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
2. Iron based Li-ion insertion materials for battery applications
Open this publication in new window or tab >>Iron based Li-ion insertion materials for battery applications
2016 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Li-ion batteries are currently the most efficient technology available for electrochemical energy storage. The technology has revolutionized the portable electronics market and is becoming a corner stone for large scale applications, such as electric vehicles. It is therefore important to develop materials in which the energy storage relies on abundant redox active species, such as iron. In this thesis, new iron based electrode materials for positive electrodes in Li-ion batteries were investigated. Lithium iron pyrophosphate (Li2FeP2O7) and two polymorphs of lithium iron sulphate fluoride (LiFeSO4F) were studied.

For Li2FeP2O7, preferred oxidation of iron with different coordination numbers within the crystal structure was studied, and six-coordinated iron was found to be oxidized preferentially at lower potentials compared to five‑coordinated iron. Electrochemical cycling resulted in structural changes of Li2FeP2O7 through an increased Li-Fe mixing in the compound, forming a metastable state during battery operation.

For tavorite LiFeSO4F, the influence of the amount of a conductive polymer (poly(3,4-ethylenedioxythiophene), or PEDOT) was studied. All the different amounts of PEDOT coating reduced the polarization significantly, but the trade-off between functionality and weight added also has to be considered. Additionally, the effect of densifying the electrodes to different degrees is reported, and was found to have a significant influence on the battery performance. Also triplite LiFeSO4F was coated with PEODT, and it was found that the electrochemical performance improved, but not to the same extent as for tavorite LiFeSO4F. The faster solid state transport of Li-ions in tavorite type LiFeSO4F possibly accounts for the difference in electrochemical performance.

Together, the results presented herein should be of importance for developing new iron based materials for Li-ion batteries.

Abstract [sv]

Av de idag tillgängliga teknologierna för elektrokemisk energilagring så har litium-jonbatterier de bästa egenskaperna när det gäller energiförluster och energilagringskapacitet. De har revolutionerat marknaden för portabel elektronik (telefoner, laptops etc.), och blir mer och mer viktiga för storskaliga tillämpningar såsom elbilar. För den typen av applikationer måste teknologin baseras på vanligt förekommande material och grundämnen, t.ex. järn.

I den här avhandlingen har järnbaserade material för den positiva elektroden hos litium-jonbatterier studerats. Olika aspekter som påverkar spänningen och effektiviteten hos elektroderna har undersökts. Ett exempel på det är hur olika omgivningar kring järnatomerna i en förening påverkar spänningen hos ett batteri. För föreningen litiumjärnpyrofosfat visade det sig att sex närmaste grannar ger lägre spänning än fem närmaste grannar till järn. Dessutom har förändringar i föreningens struktur studerats då den används i ett batteri. Den här typen av grundforskning är viktig för förståelsen av nya elektrodmaterial i Li-jonbatterier.

Ur en mer praktisk synvinkel så har elektroder baserade på en annan järnförening, litiumjärnsulfatfluorid, utvecklats. Ledningsförmågan hos dessa elektroder har förbättrats genom att belägga föreningen med ett ledande skikt, samt att mekaniskt pressa samman elektroderna genom mangling. Båda metoderna är viktiga för att tillverka välfungerande elektroder. Föreningen litiumjärnsulfatfluorid förekommer i två olika former, och en jämförelse av hur elektriskt ledande beläggningar påverkar de bägge materialen har också gjorts i den här avhandlingen.

Tillsammans visar resultaten från de olika studierna på hur man kan arbeta och tänka kring utvecklingen av nya material för litium-jonbatterier.  

Place, publisher, year, edition, pages
Uppsala University, Department of Chemistry - Ångström Laboratory, 2016. 39 p.
Keyword
Li-ion, battery, Li2FeP2O7, LiFeSO4F, PEDOT
National Category
Inorganic Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-281263 (URN)
Presentation
2016-04-12, Å2005, Ångström Laboratory, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Funder
Swedish Foundation for Strategic Research , EMI11-0028
Available from: 2016-03-22 Created: 2016-03-21 Last updated: 2016-03-22Bibliographically approved

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