Thermoelectric Phenomena in Lithium Ion Batteries and Thermogalvanic Cells

Abstract

Electrochemical cells are central in the shift towards green energy sources. Lithium-ion batteries have rapidly become one of the world’s leading energy storage technologies. Thermogalvanic cells can be used to convert waste heat to electricity or increase the energy efficiency of existing electrochemical processes. Through the work in this thesis, we have aimed to improve the understanding of transport phenomena in electrochemical cells, with a focus on thermoelectric phenomena in cells relevant to lithium-ion batteries and thermogalvanic cells. Non-equilibrium thermodynamics, experiments and molecular dynamics simulations have been used in the work.

Temperature affects the performance, capacity, safety and ageing of lithium-ion batteries. To understand this temperature dependence, we need accurate models for heat evolution and –absorption within the battery. For this we need knowledge of heat sources and sinks within the system. Reversible heat is generated or absorbed at the electrode surfaces during the reaction of electrochemical cells. In today’s thermal modelling of lithium-ion batteries, reversible heat effects are included on a cell level; in this thesis the local effects – the Peltier heats – are measured experimentally and used in a thermal model. The anode reaction leads to a temperature rise while the cathode reaction leads to a cooling of the surroundings. The local effect is usually larger than the net cell effect.

Accurate modelling of the voltage drop across an electrolyte requires detailed knowledge of coupled transport phenomena. In this thesis we have studied coupled transport using two possible sets of transport equations for two formulations (the neutral component formulation and the ionic component formulation), experiments and molecular simulations. Concentration differences, of both the salt and neutral solvent components of the electrolyte, contributes to the voltage drop across the electrolyte. Thermoelectric coupling phenomena affects the voltage drop of lithium-ion batteries significantly if a temperature difference is applied across the cell.