Impedance-based state estimation of lithium-ion batteries
Creators: name: McCarthy, Kieran
Publisher: name: University of Limerick
Year: 2022
Subjects: subject: Chemical sciences
subject: FOS: Chemical sciences

description: This thesis can be divided into two interconnecting sections. The first section is centered on the development of an internal temperature (IT) and state of health (SoH) estimator for lithium-ion batteries (LIB) through the use of online electrochemical impedance spectroscopy (EIS) measurements. Correlation analysis on the relationship between sensitive impedance variables and battery state parameters was also performed and the strength of this relationship quantified. The second section focuses on the electrochemical performance of the nanostructured Li-alloying materials silicon (Si) and germanium (Ge) nanowires (NW) for next-generation LIBs. EIS is used throughout in order to gain insights into the electrochemical properties of the novel materials. Chapters are arranged as research articles, each with an introductory summary. Chapter 3 describes the development of a novel LIB IT estimator using EIS. The proposed model used a single frequency impedance point, which displayed high sensitivity to changes in temperature while simultaneously showing low dependence to the change in state of charge (SoC) and SoH. The model was able to accurately estimate the internal temperature of commercial LIBs, over the temperature range 10 °C to 55 °C, achieving an average RMSE of 1.41 °C across 9 data sets. Chapter 4 describes the development of a SoH estimation model for LIBs. Similar to the previous chapter, the impedance at a single frequency is used to relate the fade in capacity to the increase in impedance as the battery aged. The model successfully estimated the capacity of 6 commercial batteries over 300 cycles, with an RMSE of 0.0064 Ah achieved between actual and estimated capacities. In chapter 5, sensitivity correlation analysis is performed to identify and quantify the dependence of various equivalent circuit elements and frequencies to the vital battery parameters of SoC, SoH and IT. Each parameter demonstrated high sensitivity to particular frequency ranges and parts of impedance, thus validating the intrinsic relationship each parameter has with impedance. Chapter 6 details the electrochemical behaviour of graphite (Gr), Si NW and Ge NW electrodes over a wide range of parameters including temperature, ageing and SoC. It was found that the nanostructured nature of Si NWs and Ge NWs dramatically improved their low temperature performance compared to micro-sized Gr. The fracturing and regrowth of the SEI layer during lithiation/delithiation was monitored using the charge transfer resistance acquired via EIS. Much larger variations in charge transfer resistance were observed as a function of SoC for Si NW and Ge NW electrodes, which was a result of the cracking and reformation of the solid electrolyte interface (SEI) layer caused by large volume changes during lithiation and delithiation. In chapter 7, the effect of cell configuration on the performance of Si NW, Si NW:Gr composite and Gr electrodes was explored. The effect of separator paper, electrolyte, additives and electrochemical test procedures were assessed. It was shown that the variation in cell design had a major effect on the rate and cycle performance of each material, demonstrating the key role of each component on the overall cell performance.