Non-equilibrium thermodynamics in porous media : battery degradation, and sorption and transport in porous materials

Abstract

Porous media offer many interesting problems in physics and engineering due to the interaction of phase transitions, surface effects and transport. In this thesis I examine two such problems: the degradation of lithium-ion batteries, and sorption and transport of fluids in porous materials. The dominant capacity fade mechanism in many lithium-ion batteries is the loss of cyclable lithium to a solid-electrolyte interphase layer on the surface of the negative electrode. I develop a single-particle model of this fade mechanism, based on diffusion of the reacting species through the growing layer and reaction at the surface of the active material. This analytical model is justified by comparison with a computational porous electrode model. Temperature is identified as the most important variable influencing the capacity fade rate, and the model is able to make predictions for accelerated aging tests as well as the effect of mismatched internal resistances in battery packs. The quantity of a fluid taken up by a porous material as a function of the partial pressure of the fluid relative to saturation can be used to measure the pore size distribution of the material. However, hysteresis between the wetting and drying paths complicates the interpretation of experimental results. I present a unified model of hysteresis that accounts for both single-pore and network effects, enabling the calculation of not only the pore size distribution but also a parameter measuring the connectivity between large and small pores. I then use the ideas of the model to examine drying shrinkage in hardened cement paste, demonstrating that the hysteresis in this shrinkage is primarily due to water inserted between molecular layers in calcium-silicate-hydrate. Finally, I outline a model of transport of a sorbing fluid with hysteresis, and suggest possible extensions to allow quantitative comparison with experimental results.