Modelling and simulations of energy storage devices

Abstract

Energy storage is a field which governs almost all aspects of daily life. Batteries and capacitors are utilised in devices at every scale, from mobile phones all the way up to the national grid. Efficient energy storage requires extensive research into chemistries and configurations, utilising novel materials to assemble devices. These often have a significant effect on features including the energy and power density of a cell. One such property is the pore structure of an electrode. Electrochemical Double-Layer Capacitors, also known as supercapacitors, utilise meso/microporous electrodes with a wide distribution of pore sizes. Larger pores typically facilitate ion diffusion and a high device power density, yet smaller micropores provide a large specific surface area for the electric double layer which facilitates a high device energy density. Recent advancements in post-lithium ion battery technology (such as lithium-sulphur and lithium air batteries) utilise these micropore surface areas for redox reactions. Developing accurate models of energy storage devices allows for the simulation of the processes within the cells without the need for expensive and time-consuming experimental testing. Many models have been developed in the past, however sweeping assumptions are often made with respect to the pore structure. These generally assume a uniform pore size and structure, and do not take into account the effects of different pore sizes present in a true electrode material. Pore network models are able to do this, but require complex or expensive pieces of software and a long simulation time. In this project, a novel continuum model of mass and charge transport has been developed catering for the many pore sizes in an electrode. This model allowed for the simple implementation of a pore size distribution into the simulation of the processes in several supercapacitor configurations, a lithium-sulphur battery, and two lithium-oxygen battery configurations. This novel approach involved characterising an electrode material through analysis of the pore structure and pore size distribution. A novel mathematical transient volume averaged model was developed to solve for the mass transport of species within pores of different sizes. The use of this model in each case demonstrated good agreement with experimental data and allowed for analysis of the effects of different pore structures on the activity of cells. It was found that larger pores (in the macropore region) facilitate mass transport of species, and smaller pores (in the micropore region) have a reduced rate of mass transport and a higher rate of reaction (in batteries) and stern layer formation (in supercapacitors). This novel model demonstrated the importance of modelling ion transport through multiple pore sizes of an electrode material. This model also addresses the need for more complex energy storage device simulation in a reasonable solving time without the need for expensive pieces of software.