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Electrochemical Investigation of the Reaction Mechanism in Lithium-Oxygen Batteries
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Applied Electrochemistry.ORCID iD: 0000-0001-7539-0304
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Lithium-oxygen batteries, also known as Lithium-air batteries, could possibly revolutionize energy storage as we know. By letting lithium react with ambient oxygen gas very large theoretical energy densities are possible. However, there are several challenges remaining to be solved, such as finding suitable materials and understanding the reaction, before the lithium-oxygen battery could be commercialized. The scope of this thesis is focusing on the latter of these challenges.

Efficient ion transport between the electrodes is imperative for all batteries that need high power density and energy efficiency. Here the mass transport properties of lithium ions in several different solvents was evaluated. The results showed that the lithium  mass transport in electrolytes based on the commonly used lithium-oxygen battery solvent dimethyl sulfoxide (DMSO) was very similar to that of conventional lithium-ion battery electrolytes. However, when room temperature ionic liquids were used the performance severely decreased.

Addition of Li salt will effect the oxygen concentration in DMSO-based electrolytes. The choice of lithium salt influenced whether the oxygen concentration increased or decreased. At one molar salt concentration the highest oxygen solubility was 68 % larger than the lowest one.

Two model systems was used to study the electrochemical reaction: A quartz crystal microbalance and a cylindrical ultramicroelectrode. The combined usage of these systems showed that during discharge soluble lithium superoxide was produced. A consequence of this was that not all discharge product ended up on the electrode surface.

During discharge the cylindrical ultramicroelectrodes displayed signs of passivation that previous theory could not adequately describe. Here the passivation was explained in terms of depletion of active sites. A mechanism was also proposed.

The O2 and Li+ concentration dependencies of the discharge process were evaluated by determining the reactant reaction order under kinetic and mass transport control. Under kinetic control the system showed non-integer reaction orders with that of oxygen close to 0.5 suggesting that the current determining step involves adsorption of oxygen. At higher overpotentials, at mass transport control, the reaction order of lithium and oxygen was zero and one, respectively. These results suggest that changes in oxygen concentration will influence the current more than that of lithium.

During charging not all of the reaction product was removed. This caused an accumulation when several cycles was examined. The charge reaction pathway involved de-lithiation and bulk oxidation, it also showed an oxygen concentration dependence.

Abstract [sv]

Litiumsyrebatteriet, även känt som litiumluftbatteriet, kan potentiellt revolutionera vårt förhållande till energilagring. Genom att låta litium reagera med syrgas från luften kan teoretiskt höga energitätheter uppnås. Dock så behöver många problem lösas, så som att hitta lämpliga elektrod- och elektrolytmaterial samt att få en ökad förståelse för reaktionsmekanismen, innan litiumsyrebatteriet kan kommersialiseras. Den här avhandlingen behandlar de sistnämnda av dessa problem.

För att ett batteri ska kunna leverera hög effekttäthet och energieffektivitet krävs en effektiv jontransport mellan elektroderna. Här utvärderades masstransporten hos flera olika elektrolyter. Resultatet visade att masstransporten av litium i en litiumsyrebatterielektrolyt (baserad på dimetylsulfoxid (DMSO)) är likvärdig med en konventionell litiumjonbatterielektrolyt. När elektrolyter baserade på jonvätskor användes uppvisades väldigt stora energiförluster.

När litiumsalt tillsattes påverkades lösligheten av syre i DMSO-baserade elektrolyter. Vilken sorts litiumsalt som användes påverkade om lösligheten av syre ökade eller minskade. Vid en saltkoncentration på en molar var den högsta syrelösligheten 68 \% större än den lägsta.

Två olika modellsystem används för att studera den elektrokemiska reaktionen: En elektrokemisk kvartskristallmikrovåg och en cylindrisk ultramikroelektrod. Vid kombinerad användning av dessa system påvisades att löslig litiumsuperoxid bildades vid urladdningen. Följden av detta blev att endast delar av urladdningsprodukten hamnade på elektroden.

Vid urladdning visade ultramikroelektroderna tecken på passivering som inte kunde beskrivas av tidigare teori. Här föreslås att passiveringen uppstår på grund av en blockering av de aktiva säten där reaktionen fortskrider. För denna process föreslås även en detaljerad mekanism.

Urladdningsprocessens koncentrationsberoende utvärderades genom att bestämma reaktionsordningen för syre och litium under kinetisk- och masstransport kontroll. Under kinetisk kontroll fanns inga heltalsreaktionsordningar, för syre var reaktionsordningen nära 0.5 vilket föreslår att det reaktionssteg som bestämmer strömstorleken innefattar en adsorption av syre. Vid högre överpotentialer, då systemet var under masstransportkontroll, var reaktionsordningarna för litium och syre noll respektive ett. Detta föreslår att ändringar i syrekoncentration påverkar strömmen betydligt mer än vad det gör för litium.

Under uppladdning kunde inte all reaktionsprodukt avlägsnas från elektroden. Detta ledde till en ackumulation då flera cykler studerades. Uppladdningens delsteg innefattade en delitiering följt av en oxidation av reaktionsproduktbulken. Denna process uppvisade även ett syrekoncentrationsberoende.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2017. , 64 p.
Series
TRITA-CHE-Report, ISSN 1654-1081
Keyword [en]
Lithium-Oxygen Batteries, Lithium Mass Transport, Oxygen Solubility, Reaction Rate Law, Reaction Mechanism, Electrode Passivation, Ultramicroelectrodes, Electrochemical Quartz Crystal Microbalance, EQCM
National Category
Other Chemical Engineering
Research subject
Chemical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-217533ISBN: 978-91-7729-614-0 (print)OAI: oai:DiVA.org:kth-217533DiVA: diva2:1156782
Public defence
2017-12-18, K1, Teknikringen 56, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Foundation for Strategic Research , EM11-0028
Note

QC 20171114

Available from: 2017-11-14 Created: 2017-11-14 Last updated: 2017-11-14Bibliographically approved
List of papers
1. The effect of O2 concentration on the reaction mechanism in Li-O2 batteries
Open this publication in new window or tab >>The effect of O2 concentration on the reaction mechanism in Li-O2 batteries
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2017 (English)In: Journal of Electroanalytical Chemistry, ISSN 0022-0728, E-ISSN 1873-2569, Vol. 797, 1-7 p.Article in journal (Refereed) Published
Abstract [en]

The promising lithium-oxygen battery chemistry presents a set of challenges that need to be solved if commercialization is ever to be realized. This study focuses on how the O2 reaction path is effected by the O2 concentration in the electrolyte. An electrochemical quartz crystal microbalance system was used to measure current, potential, and change in electrode mass simultaneously. It is concluded that the mass reversibility is O2 concentration dependent while the coulombic efficiency is not. The mass reversibility is higher at low O2 concentration meaning that more of the deposited Li2O2 is removed during oxidation in relation to the amount deposited during reduction. The first step of the reduction is the formation of soluble LiO2, which is then either reacting further at the electrode or being transported away from the electrode resulting in low current efficiency and low deposited mass per electrons transferred. During the oxidation, the first step involves de-lithiation of Li2O2 at low potential followed by bulk oxidation. The oxidation behavior is O2 concentration dependent, and this dependence is likely indirect as the O2 concentration effects the amount of discharge product formed during the reduction. The O2 concentration at different saturation pressures was determined using a mass spectrometer. It was found that the electrolyte follows Henry's law at the pressures used in the study. In conclusion, this study provides insight to the O2 concentration dependence and the preferred path of the O2 electrochemical reactions in lithium-oxygen batteries.

Place, publisher, year, edition, pages
Elsevier, 2017
Keyword
Atomic force microscope, Electrochemical quartz crystal microbalance, Mass spectrometer, Non-aqueous lithium-oxygen battery, O2 concentration dependence, O2 saturation concentration, Reaction mechanism
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-209523 (URN)10.1016/j.jelechem.2017.05.005 (DOI)000404696900001 ()2-s2.0-85019102835 (Scopus ID)
Funder
Swedish Foundation for Strategic Research , EM11-0028
Note

QC 20170620

Available from: 2017-06-20 Created: 2017-06-20 Last updated: 2017-11-14Bibliographically approved

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