Toward a deeper understanding of concentrated electrolyte effects in redox flow batteries

Thesis

The rapid uptake of wind and solar power has created an urgent need for long-duration, grid-scale energy-storage technologies that are efficient, safe, and economically scalable. Redox flow batteries (RFBs) satisfy many of these requirements, yet their commercial adoption is hampered by poor durability, which in part owes to gaps in the fundamental understanding of ion transport in the concentrated electrolytes favored for high energy density. This dissertation delivers an integrated experimental–theoretical investigation that traces performance bottlenecks from the cell-scale symptoms of capacity fade to the micro-scale physics that govern ion motion in complex fluids.

Chapters two and three diagnose and mitigate the principal durability challenge in non-aqueous RFBs: membrane fouling. Using galvanostatic cycling, impedance spectroscopy, and a novel “canary-cell” protocol, I show that pore clogging — not electrolyte decomposition — dominates capacity decay in vanadium acetylacetonate systems. Flow-field redesign and active reservoir rebalancing suppress concentration gradients across the separator and extend cycle life by an order of magnitude.

Motivated by these findings, Chapter four revisits multicomponent transport theory. I recast the Onsager—Stefan—Maxwell (OSM) equations in a salt–charge coordinate system, clarifying how electroneutrality, thermodynamic non-ideality, and ion-ion friction govern ion transport and pinpointing the assumptions that collapse the OSM framework into the familiar Nernst—Planck form. The resulting hierarchy of sub-models, each invoking only the transport and thermodynamic parameters it needs, resolves coupled concentration and electrostatic-potential gradients in space and time, enabling practical prediction of ionic fluxes.

Chapter five closes the property loop by measuring the viscosity, density, sound speed, and compressibility of LiPF6 in mixed-carbonate solvents over a broad concentration range. These constitutive relations, coupled with the new transport formalism, enable predictive simulation of concentrated-electrolyte behavior under extreme pressure and temperature variations. Although chemically distinct, both lithium-ion and non-aqueous RFB electrolytes combine rigid-ring solutes with flexible linear co-solvents, indicating that mixing volumes, partial-molar compressibilities, and related Gibbs free-energy derivatives follow generally transferable trends.

Collectively, the thesis integrates separator diagnostics, flow-field engineering, advanced transport modeling, and comprehensive property characterisation to build a unified picture of concentrated-electrolyte effects in RFBs. The resulting insights shorten the path from laboratory discovery to field deployment, guiding the design of more durable, efficient, and cost-competitive flow-battery systems essential for a decarbonised electricity grid.

Authors: Rungta, R

• DOI: 10.5287/ora-eyqn8dv1b
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English