Applying ab initio and statistical methods to PEDOT with AlCl4- for enhanced theoretical understanding and energy storage applications

Abstract

Inspired by the use of poly(3,4-ethylenedioxythiophene (PEDOT) as the positive electrode in a battery with an aluminium negative electrode and AlCl3-[EMIm]Cl electrolyte, this thesis aims to carry out a comprehensive investigation of single oligomers of PEDOT with adsorbed AlCl4- anions via ab initio and statistical methods. It spans several major research fields: post-lithium batteries, fundamental conducting polymer theory, computational methods for studying conducting polymers, and statistical mechanics, particularly applying to the behaviour of conducting polymers in an energy storage context. The work therefore paves the way to better predictions of the behaviour of conducting polymer electrodes in energy storage applications and makes a broad contribution to conducting polymer theoretical understanding and methods. A full review of non-aqueous aluminium batteries is provided, leading into a detailed methodology review of computational approaches used for conducting polymers. Ab initio molecular dynamics is used to assess the robustness of the DFT ground states. The electronic structure of the PEDOT/AlCl4- system is comprehensively investigated. The findings are discussed in the context of the evolving models describing conducting polymers, with Coulombic repulsion between anions found to be a dominant influence on the electronic structure of the system, leading to a new hypothesis relating to conductivity at high anion concentrations. There is a clearly observed tendency for anions to locate over the sulfur atoms, which is robustly tested via density functional theory and ab initio molecular dynamics. Taking advantage of the ability to treat the sulfur atoms as anion adsorption lattice sites, statistical methods including the grand canonical partition function, cluster expansion and Monte Carlo methods are developed for the system. Validated against a small 6-PEDOT system for which all configurations are known, the method is extended to the 12-PEDOT system. Coulombic repulsion is again found to be the dominant factor defining the configurational energies, with the PEDOT screening anions on opposite sides of the oligomer. Ultimately, the method is shown to work well for single PEDOT oligomers with AlCl4- anions. The results are compared to the experimental battery results. The chemical potential vs. anion concentration behaviour suggests that the anion concentration range achievable in PEDOT or other conducting polymers is indeed likely to be limited to between 0.05 and 0.4, in contrast to experimental work where anion concentrations of 1 had been estimated. This finding should help others carry out realistic feasibility assessments of future conducting polymer prototype energy storage devices.

More information

Identifiers

ORCID for Benjamin Craig: orcid.org/0000-0001-6948-4320

ORCID for Carlos Ponce De Leon Albarran: orcid.org/0000-0002-1907-5913

ORCID for Chris Skylaris: orcid.org/0000-0003-0258-3433

Catalogue record

Export record

Altmetrics