Impedance-based state estimation of lithium-ion batteries

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.  </p

Review—Use of impedance spectroscopy for the estimation of Li-ion battery state of charge, state of health and internal temperature

The rapid adoption of electric vehicles (EVs) and the evolving needs of portable electronic devices has intensified the need for enhanced state diagnosis of Li-ion batteries (LIBs). As the applications for LIBs continue to grow, so too does their operational requirements; ranging from faster charging and improved safety to optimized energy control and extended lifespan. In order to keep pace with the growing requirements of LIBs, improvements in the monitoring of battery states must be achieved. Although electrochemical impedance spectroscopy (EIS) has existed since the 1960's, its potential as a diagnosis tool has only received widespread attention in recent years. In this paper, a detailed review on the applicability of impedance measurements for the estimation of the vital battery parameters of state of charge (SOC), state of health (SOH) and internal temperature (IT) has been performed

Highlighting the importance of full-cell testing for high performance anode materials comprising Li alloying nanowires

Herein, the electrochemical performance of directly grown Ge nanowire anodes in full-cell Li-ion configurations (using lithium cobalt oxide cathodes) are examined. The impacts of voltage window, anode/cathode balancing and anode preconditioning are assessed. The cells had a useable upper cutoff of 3.9 V, with a higher voltage cutoff of 4.2 V shown by SEM analysis to lead to Li plating on the anode surface. The rate performance of Ge NW anodes was shown to be boosted within full-cells compared to half-cells, meaning that existing studies may underestimate the rate performance of alloying mode anode materials if they are only based on half-cell investigations. The capacity retention of the full-cells is lower compared to equivalent half-cells due to progressive consumption of cyclable Li. This phenomenon is demonstrated using a parallel anode and cathode delithiation approach that could be extended to other full-cell systems. The findings stress the importance of testing promising anode materials within full-cell configurations, to identify specific capacity fade mechanisms that are not relevant to half-cells and aid the development of higher energy density storage systems

Direct growth of Si, Ge, and Si–Ge heterostructure nanowires using electroplated Zn: an inexpensive seeding technique for Li-Ion alloying anodes

A scalable and cost-effective process is used to electroplate metallic Zn seeds on stainless steel substrates. Si and Ge nanowires (NWs) are subsequently grown by placing the electroplated substrates in the solution phase of a refluxing organic solvent at temperatures >430 °C and injecting the respective liquid precursors. The native oxide layer formed on reactive metals such as Zn can obstruct NW growth and is removed in situ by injecting the reducing agent LiBH4. The findings show that the use of Zn as a catalyst produces defect-rich Si NWs that can be extended to the synthesis of Si–Ge axial heterostructure NWs with an atomically abrupt Si–Ge interface. As an anode material, the as grown Zn seeded Si NWs yield an initial discharge capacity of 1772 mAh g−1 and a high capacity retention of 85% after 100 cycles with the active participation of both Si and Zn during cycling. Notably, the Zn seeds actively participate in the Li-cycling activities by incorporating into the Si NWs body via a Li-assisted welding process, resulting in restructuring the NWs into a highly porous network structure that maintains a stable cycling performance