Application of concentrated solution theory and core potential to ion exchange media

Thesis

Simple electrolytic systems are studied under the framework of Newman's concentrated-solution theory without making an explicit assumption of electroneutrality. To ensure thermodynamic consistency in non-electroneutral systems, Goyal and Monroe's core-potential theory is incorporated.

Three electrolytic solutions are investigated first in this thesis: (1) the binary electrolyte, (2) the three-ion ionic liquid, and (3) the binary salt in two neutral solvents. Corrections to electrochemical-potential differentials (dμ_i) are derived by combining core-potential theory with the Boltzmann distribution, in accordance with Goyal and Monroe's treatment. The new dμ_i's —which satisfy the fundamental Gibbs–Duhem relation universally— can be directly employed to describe systems and situations in which diffusion takes place, regardless of the charge state of the system.

Using the new dμ_i expressions in practice requires knowing the functional forms of concentration-dependent thermodynamic factors. The liquid-junction potential accessed by the concentration-cell experiment is investigated as the main thermodynamic-factor characterisation technique in this thesis. Thus measured liquid-junction potential for the EMC:EC:LiPF6 electrolyte —a commonly used cosolvent electrolyte in conventional lithium-ion batteries— is parameterised as an illustrative example. This exercise reveals that overpotentials arise from solvent polarisation; thus, it is important to distinguish the two solvents in high-fidelity battery models for an accurate description of long-term battery operation.

The theoretical endeavour so far is then extended to a charged membrane submerged in a binary electrolyte, which shares aspects of the three-ion ionic liquid (having two neutral entities with a shared ion) and the binary salt in two neutral solvents (having four species). Corrections to dμ_i's, as well as thermodynamic-factor expressions, are derived. Furthermore, concentration dependences of two Stefan–Maxwell diffusivities associated with the charged membrane end-group are derived for Nafion 117 in aqueous sulfuric acid. This is done by revisiting Verbrugge and Hill's radio-tracing experiment and the ionic-conductivity measurement. The resulting Stefan–Maxwell diffusivities are then utilised to estimate the macroscopic transport properties, illustrating the usefulness of having access to the full suite of concentration-dependent Stefan–Maxwell diffusivities.

Authors: Jung, T

• DOI: 10.5287/ora-mvz4e75py
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: core-potential theory; diffusion in non-electroneutral regions; diffusion across membrane; concentrated solution theory; thermodynamic factors
• Subjects: Diffusion; Thermodynamics; Membrane; Lithium ion batteries; Electrochemistry