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Going beyond sweep voltammetry: Alternative approaches in search of the elusive electrochemical stability of polymer electrolytes
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Structural Chemistry.ORCID iD: 0000-0002-2004-5869
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Structural Chemistry.ORCID iD: 0000-0002-3374-2276
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Structural Chemistry.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Structural Chemistry.
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2021 (English)In: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, Vol. 168, no 10, article id 100523Article in journal (Refereed) Published
Abstract [en]

Solid polymer electrolytes (SPEs) are promising candidates for solid-state lithium-ion batteries. Potentially, they can be used with lithium metal anodes and high-voltage cathodes, provided that their electrochemical stability is sufficient. Thus far, the oxidative stability has largely been asserted based on results obtained with sweep voltammetry, which are often determined and reliant on arbitrary assessments that are highly dependent on the experimental conditions and do not take the interaction between the electrolyte and the electrode material into account. In this study, alternative techniques are introduced to address the pitfalls of sweep voltammetry for determining the oxidative stability of SPEs. Staircase voltammetry involves static conditions and eliminates the kinetic aspects of sweep voltammetry, and coupled with impedance spectroscopy provides information of changes in resistance and interphase layer formation. Synthetic charge–discharge profile voltammetry applies the real voltage profile of the active material of interest. The added effect of the electrode active material is investigated with a cut-off increase cell cycling method where the upper cut-off voltage during galvanostatic cycling is gradually increased. The feasibility of these techniques has been tested with both poly(ethylene oxide) and poly(trimethylene carbonate) combined with LiTFSI, thereby showing the applicability for several categories of SPEs.

Place, publisher, year, edition, pages
2021. Vol. 168, no 10, article id 100523
National Category
Materials Chemistry Polymer Chemistry
Research subject
Chemistry with specialization in Materials Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-440872DOI: 10.1149/1945-7111/ac2d8bISI: 000709097900001OAI: oai:DiVA.org:uu-440872DiVA, id: diva2:1546240
Funder
EU, European Research Council, 771777Available from: 2021-04-21 Created: 2021-04-21 Last updated: 2023-03-05Bibliographically approved
In thesis
1. Exploring the Frontiers of Polymer Electrolytes for Battery Applications: From Surface to Bulk
Open this publication in new window or tab >>Exploring the Frontiers of Polymer Electrolytes for Battery Applications: From Surface to Bulk
2021 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Lithium-ion batteries have dominated the market since their inception in 1991 due to their unparalleled energy and power densities, but are now faced with new challenges. Growing demand for battery materials for energy intense applications and large-scale interim energy storage have emphasized the need for safe and sustainable battery electrolytes. In this context, non-flammable solid polymer electrolytes (SPEs) are a promising alternative to address the shortcomings of conventional liquid electrolytes. Despite its significance, little research has thus far been devoted to understanding the electrochemical stability of SPEs under the harsh conditions exerted by next-generation electrode materials.

In this thesis, the stability and ramifications of interfaces in polycarbonate- and polyester-based SPEs have been investigated. The polycarbonate exhibited severe degradation upon contact with lithium compared to its ester counterpart. Volatile species stemming from polycarbonate and salt decomposition were observed independent of irreversible current response, thus also highlighting the limitations of voltammetry techniques to determine the electrochemical stability. Two novel techniques were thus devised to evaluate electrochemical stability of SPEs under more realistic conditions. Characterization of the electrode−polyester interface revealed formation of highly resistive interfacial layers composed of polymer, salt and impurity derivatives. The emergence of a detrimental resistance emanating from the polymer−polymer interface was also observed, thus identifying a crucial hurdle for double-layer SPEs as a strategy to extend the stability window.

The application of polycarbonate/polyester-based polymer electrolytes for sodium-ion batteries was also studied. Sodium is far more abundant than lithium, and thereby an excellent chemistry platform to develop new sustainable battery materials. The polycarbonate exhibited an exceptional ability to dissolve large quantities of sodium salt without compromising the mechanical stability. Spectroscopic and thermal measurements revealed the emergence of an alternative ionic transport mechanism at concentrations within the polymer-in-salt regime, which was decoupled from the segmental motion of the polymer chains. By incorporating flexible polyester moieties in polycarbonates, an SPE with better transport properties compared to its individual subunits, and polyether counterparts, was obtained. Optimal salt concentration in this copolymer was dependent on the degree of crystallinity, determined by the portion of polyester. Finally, the practical application of these polymer electrolytes was demonstrated in solid-state sodium-ion batteries.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2021. p. 67
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 2044
Keywords
Lithium-ion batteries, solid-state polymer electrolytes, electrochemical stability window, interfaces, ionic conductivity, polycarbonate, polyester, sustainability, sodium-ion
National Category
Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-440905 (URN)978-91-513-1214-9 (ISBN)
Public defence
2021-06-11, Room Å2001, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2021-05-19 Created: 2021-04-21 Last updated: 2021-06-23
2. The Art of Cycling – Polymer Electrolytes at Extreme Conditions
Open this publication in new window or tab >>The Art of Cycling – Polymer Electrolytes at Extreme Conditions
2023 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

With the rapid development of batteries for applications like electric vehicles and energy storage devices, it is essential to design and develop batteries with improved safety, long cycle life, and high energy density. To achieve this goal, the development and improvement of solid-state batteries, containing solid polymer electrolytes, is a promising solution. 

The interest in polymer electrolytes is primarily owed to their proposed compatibility with high temperatures and reactive electrodes, such as metallic lithium, and their ability to withstand higher temperatures than traditional liquid electrolytes. Cycling polymer electrolytes at high temperature and with high-voltage cathodes, such as lithium-nickel-manganese-cobalt-oxide (NMC) involves a combination of high chemical, electrochemical, and mechanical stability, as well as the understanding of how to achieve these properties.

This thesis provides an overview of some challenges and possibilities of cycling batteries with polymer electrolytes at high temperatures and with high-voltage cathodes. With a focus on the stability of the polymer electrolyte, the effect of changing the polymer host material, the electrolyte salt, and the introduction of additives for enhanced mechanical stability or electrochemical stability, were all evaluated by both standard techniques and techniques developed for polymer electrolytes. 

Long-term cycling at high temperature was achieved for a poly(ε-caprolactone-co-trimethylene carbonate) (PCL-PTMC) electrolyte by crosslinking additives that increase the mechanical stability of the polymer electrolyte; however, the cycling with high-voltage cathodes also required a high electrochemical stability of the polymer electrolyte. With the techniques developed herein, such as cut-off increase cell cycling, the electrochemical stability of PCL-PTMC was evaluated. By introducing zwitterionic additives to PCL-PTMC, the cycling performance with NMC was enhanced and the enhancement proved to stem from prevention of electrolyte salt decomposition. Finally, by changing the electrolyte salt, it was found that cycling with NMC was possible for PCL-PTMC below its oxidative degradation potential, as long as the electrolyte had an ionic conductivity that was high enough. By utilizing additives, the long-term stability and electrochemical stability toward NMC was also improved. 

Overall, cycling solid polymer electrolytes at high temperatures and with high-voltage cathodes presents a unique set of challenges, which require that the electrochemical stability of the electrolyte is accurately described, and that the following properties are high: ionic conductivity, electrochemical and mechanical stability; all of which can be improved by utilizing additives in the polymer electrolyte. 

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2023. p. 80
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 2247
Keywords
Solid polymer electrolytes, Lithium ion batteries, Electrochemical stability, Mechanical stability, Ionic conductivity, Additives, Polycarbonate, Polyester, Polyketone
National Category
Materials Chemistry
Research subject
Chemistry with specialization in Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-497868 (URN)978-91-513-1734-2 (ISBN)
Public defence
2023-04-21, Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2023-03-30 Created: 2023-03-05 Last updated: 2023-03-30

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Hernández, GuiomarSångeland, ChristoferJohansson, IsabellBrandell, DanielMindemark, Jonas

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