Aqueous Electrolytes for High-Voltage Batteries and Supercapacitors

Thèse n° 8287 Aqueous electrolytes for high-voltage batteries and supercapacitors Présentée le 23 juillet 2020 à la Faculté des sciences et techniques de l’ingénieur Unité de rattachement pour scientifiques IMX Programme doctoral en science et génie des matériaux pour l’obtention du grade de Docteur ès Sciences par David REBER Acceptée sur proposition du jury Prof. P. Hoffmann, président du jury Prof. F. Nüesch, Dr C. Battaglia, directeurs de thèse Dr K. Xu, rapporteur Prof. P. Broekmann, rapporteur Prof. S. Haussener, rapporteuse 2020 Abstract Lithium-ion batteries are widely implemented as energy storage devices due to their high energy density and low cost. They enabled modern portable electronics and electric vehicles, and are key to manage the integration of intermittent renewable electricity sources, such as solar and wind, into the power grid and decentralized microgrids. The use of flammable organic electrolytes however raises safety concerns, particularly for large-scale stationary batteries. Furthermore, supply chain risks and the use of critical raw materials are a major challenge for the ongoing battery revolution. Aqueous electrolytes offer promising properties, as they are inherently non-flammable and potentially more cost effective. Combined with electrode materials based on abundant raw materials, aqueous batteries are among the most promising candidates to replace current battery chemistries relying on critical cobalt and flammable organic electrolytes. However, due to the narrow electrochemical stability window of water (practically ca. 1.5 V), aqueous batteries have not yet met the energy density to be serious competitors to current lithium- ion battery technology. The goal of this thesis was to develop aqueous electrolytes with enhanced electrochemical stability, thus enabling new high-voltage cell chemistries. Exploring aqueous electrolytes with high salt concentrations, so-called water-in-salt electrolytes, we significantly increased the electrochemical stability and developed highly stable 2 V class aqueous sodium-ion batteries. Operating near the solubility limit of the electrolyte salt, crystallization turned out to be a major challenge for water-in-salt electrolytes. In this thesis, we demonstrated that adding highly asymmetric anions into the solution can effectively suppress crystallization, allowing stable operation of saturated electrolytes at temperatures as low as -10 °C. We further exam- ined the impact of anion asymmetry on the local solution structure and dynamics using molecular dynamics simulations, relating the enhanced supercooling to sustained rotational mo- tion of the asymmetric anion. Further, we demonstrate that the anion has a crucial impact on solution structure and cell performance. We establish that high salt concentration alone does not guarantee high- voltage stability and that due to the limited solubility of most sodium salts new approaches are needed. We provide guidelines for future work and the evaluation of experimental data in order to spur progress in the field of aqueous batteries. Keywords: Aqueous electrolytes, water-in-salt, anion asymmetry, supercooling, anion-specific effects i Zusammenfassung Lithium-Ionen-Batterien konnten sich dank ihrer hohen Energiedichte und der tiefen Kos- ten als gängige Energiespeicher etablieren. Sie ermöglichten den Durchbruch der mobilen Elektronik und elektrischer Autos und sind der Schlüssel zur erfolgreichen Integration von er- neuerbaren Energien wie Wind- oder Solarkraft ins Übertragungsnetz und kleinere delokali- sierte Netzwerke. Besonders für grosse Installationen aber ist die Nutzung brennbarer organi- scher Elektrolyte ein Sicherheitsrisiko und die Abhängigkeit von kritischen Rohmaterialien ist eine Herausforderung für die fortlaufende Batterierevolution. Wässrige Elektrolyte haben vielversprechende Eigenschaften wie Nichtbrennbarkeit und potenziell tiefe Kosten. In Kombination mit häufig vorkommenden Elektrodenmaterialien ge- hören wässrige Batterien zu den erfolgversprechendsten Kandidaten um momentan gängige Batterien, die auf Kobalt und brennbaren Elektrolyten basieren, abzulösen. Bisher leiden was- ser-basierte Batterien aufgrund des kleinen elektrochemischen Stabilitätsfensters wässriger Elektrolyte (in der Praxis ca. 1.5 V) aber unter zu tiefer Energiedichte um mit derzeitigen Bat- terien zu konkurrieren. Das Ziel dieser Thesis war die Entwicklung von wässrigen Elektrolyten mit erhöhter elekt- rochemischer Stabilität um damit Zellchemien mit hoher Spannung zu ermöglichen. Mittels hochkonzentrierter, sogenannter Water-in-Salt Elektrolyte konnten wir die Spannungstole- ranz deutlich erhöhen und wässrige Natriumbatterien in der 2 V Klasse entwickeln. Das Aus- kristallisieren dieser gesättigten Elektrolyte stellte sich als eine der grössten Herausforderun- gen während dieses Projekts heraus. Mittels asymmetrischer Anionen konnten wir die Kristallisierung unterdrücken und so Batterien bei Temperaturen unter null Grad betreiben. Mittels Molekulardynamik-Simulationen konnten wir das Unterkühlungsverhalten der Elekt- rolyte mit anhaltender Bewegung der Anionen-Seitenketten erklären. Für den Einfluss der Anionen auf die Lösungsstruktur konzentrierter Elektrolyte und damit die Stabilität der Batterien konnten wir klare Richtlinien entwickeln. Wir zeigen auf, dass hohe Salzkonzentration nicht zwangsläufig hohe elektrochemische Stabilität garantiert und besprechen Möglichkeiten für weitere Forschungsarbeiten sowie einen Leitfaden zur Auswer- tung gängiger Experimente. Stichwörter: Wässrige Elektrolyte, Water-in-Salt, Anionen-Asymmetrie, Unterkühlung, Anio- nen-spezifische Effekte iii Acknowledgments This thesis was conducted in the Laboratory Materials for Energy Conversion at Empa, the Swiss Federal Laboratories for Materials Science & Technology under the supervision of head of laboratory Dr. Corsin Battaglia. Prof. Frank Nüesch acted as academic supervisor at EPFL in the Department Materials Science and Engineering. I would like to thank them for the opportunity to do this work and their guidance and support over the past four years. I am very grateful to Dr. Corsin Battaglia for the valuable discussions, particularly on how to present and convey my results, and the freedom to explore what seemed interesting at any point. I highly appreciate the opportunities given to me to foster my network at conferences, workshops, and my stay in Tokyo, as well as the chance to take responsibilities for equipment and the battery-testing lab. Further, I am especially grateful to Dr. Ruben-Simon Kühnel who taught me the tricks of the trade and closely advised me. His extensive knowledge about electro- chemistry, specifically applied to electrochemical energy storage and electrolytes, has made this thesis possible. I would like to thank all the team members, Zivis, Bachelor- and Master students at Empa for the good times, with special thanks going to the first generation of 501-PhDs, Léo, Fran- cesco, Marie-Claude, thank you for the warm welcome, and Ryo, Daniel and Max, who were great additions to our crew. We should have gone to Sicily earlier. Thank you to Prof. Atsuo Yamada for welcoming me in his laboratory at the University of Tokyo, and Dr. Norio Takenaka for teaching me as much as possible in this short time. It was an enriching experience and I greatly enjoyed my stay. I would also like to thank Prof. Patrik Hoffmann, Dr. Kang Xu, Prof. Sophia Haussener, and Prof. Peter Broekmann for taking the time to review and evaluate my PhD thesis. Finally, thank you to my friends and family for the great support, especially to my partner in crime Nathalie. v Abbreviations AC Activated carbon BETI Bis(pentafluoroethanesulfonyl)imide CE Counter electrode CV Cyclic voltammetry DSC Differential scanning calorimetry ESW Electrochemical stability window FSI Bis(fluorosulfonyl)imide FTFSI (fluorosulfonyl)(trifluoromethanesulfonyl)imide GCPL Galvanostatic cycling with potential limitations HCF Hexacyanoferrate HER Hydrogen evolution reaction LIB Lithium-ion battery LSV Linear sweep voltammetry MD Molecular dynamics NIB Sodium-ion battery NMP N-methyl-2-pyrrolidone NTP NaTi2(PO4)3 NVPOF Na3(VOPO4)2F OER Oxygen evolution reaction OTf Trifluoromethanesulfonate PTFSI (pentafluoroethanesulfonyl)(trifluoromethanesulfonyl)imide RDF Radial distribution function RE Reference electrode RT Room temperature SEI Solid-electrolyte interphase SHE Standard hydrogen electrode SS Stainless steel TFSI Bis(trifluoromethanesulfonyl)imide TL Liquidus temperature WE Working electrode vii Contents 1 Introduction ................................................................................................................. 1 1.1 Lithium-ion Batteries ............................................................................................ 3 1.2 Supercapacitors and the Electrochemical Double Layer ..................................... 6 1.3 Aqueous Electrolytes ............................................................................................ 8 1.4 Highly Concentrated Aqueous Electrolytes........................................................ 10 1.5 Aim of the Thesis ................................................................................................ 13 2 Methods ..................................................................................................................... 21 2.1 Electrochemical Measurements ......................................................................... 21 2.2 Electrolyte Properties ........................................................................................

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