This study highlights the importance of the anion in the electrochemical uptake of lithium by metal phosphides. It is shown through a variety of <em>in-situ</em> and <em>ex-situ</em> analytical techniques that the redox active centers in three different systems (MnP<i>₄</i>, FeP<i>₂</i>, and CoP<i>₃</i>) are not necessarily cationic but can rest almost entirely upon the anionic network, thanks to the high degree of covalency of the metal-phosphorus bond and strong P-character of the uppermost filled electronic bands in the phosphides. The electrochemical mechanism responsible for reversible Li uptake depends on the transition metal, whether a lithiated ternary phase exists in the phase diagram with the same M:P stoichiometry as the binary phase, and on the structure of the starting phase. When both binary and lithiated ternary phases of the transition metal exist, as in the case of MnP<i>₄</i> and Li<i>₇</i>MnP<i>₄</i>, a semi-topotactic phase transformation between binary and ternary phases occurs upon lithium uptake and removal. When only the binary phase exists two different behaviours are observed. In the FeP<i>₂</i> system, lithium uptake leads to the formation of an amorphous material in which short-range order persists; removal of lithium reforms some the long-range order bonds. In the case of CoP<i>₃</i>, lithium uptake results in phase decomposition to metallic cobalt plus lithium triphosphide, which becomes the active material for the subsequent cycles.

Lithium iron phosphate (LiFePO4) has been synthesized by hydrothermal synthesis using pyrrole as an efficient reducing agent. The oxidized Fe3+ in the system reacts with pyrrole that can form polypyrrole (PPy) to generate Fe2+. The PPy can also be a carbon source for further calcination. The observations of scanning electron microscope (SEM) and transmission electron microscope (TEM) show that the particle size of LiFePO4 is around 500 nm and a layer of carbon coats on LiFePO4. The chemical composition of the LiFePO4 was characterized by elemental analysis (EA) and inductively coupled plasma mass spectroscopy (ICP/MS). The results of TEM and X-ray diffraction (XRD) show the structure of LiFePO4 is orthorhombic olivine. Raman and X-ray photoelectron spectroscopy (XPS) results indicate that pyrrole as a reducing agent prevents the impurity of Fe3+ formation and the resulting polypyrrole plays a role as carbon source. The calcination of LiFePO4 greatly affects the energy density. In addition, the carbon contain in the LiFePO4 powder is controllable using the addition of Fe3+ to enhance the electrical conductivity. Moreover, the electrochemical results show the energy capacity of the hydrothermal LiFePO4 is 152 mAh g−1. The LiFePO4 has a better rate discharge capability compared with LiFePO4 synthesized with ascorbic acid as a reducing agent.

Lithium iron phosphate (LiFePO4) is prepared by hydrothermal process using Fe(III) as precursor and pyrrole as an efficient reducing agent. The Fe(III) precursor in the system reacts with pyrrole to generate polypyrrole (PPy) and reduce Fe(III) to Fe(II). The different molar ratio Fe(III) polymerize different content of PPy and PPy can also be a carbon source for further calcination. The structural and morphological properties of LiFePO4 powder were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), and a transmission electron microscope (TEM). The XRD and TEM results demonstrate that LiFePO4 powder has an orthorhombic olivine-type structure with a space group of Pnma. The SEM and TEM results show that the particle size of LiFePO4 is around 200 nm and a layer of carbon coats on LiFePO4. The chemical composition of the LiFePO4 powder was characterized by elemental analysis (EA) and inductively coupled plasma/mass spectroscopy (ICP/MS). Raman and X-ray photoelectron spectroscopy (XPS) results indicate that pyrrole as a reducing agent reduces and prevents the formation of Fe(III) impurity and the resulting PPy plays a role as carbon source. Among the synthesized cathode materials, LiFePO4 synthesized using 5% molar ratio of Fe(III) and subsequent calcinations of 600 ¢XC shows the best electrochemical property with an discharge capacity of 160 mAhg−1 close to its theoretical capacity 170 mAh g−1 at 0.2 C rate. Using 10% molar ratio of Fe(III), and the discharge capacity of LiFePO4 at 10 C rate reaches 106 mAhg−1.

Interest in high performance portable energy devices for electronics and electric vehicles is the basis for a significant level of activity in battery research in recent history. Li-ion batteries are of particular interest due to their high energy density, decreasing cost, and adaptable form factor. A common goal of researchers is to develop new materials that will lower the cost and weight of Li-ion batteries while simultaneously improving the performance. There are several approaches to facilitate improved battery system-level performance including, but not limited to, the development of new material structures and/or chemistries, manufacturing techniques, and cell management. The performed research sought to enhance the understanding of structure-property relationships of carbon-containing composite anode materials in a Li-ion cell through extensive materials and anode performance characterization. The approach was to focus on the development of new electrode material designs to yield higher energy and power characteristics, as well as increased thermal and electrical conductivities or mechanical strength, using techniques that could be scaled for large volume manufacturing. Here, three different electrode architectures of nanomaterial composites were synthesized and characterized. Each electrode structure consisted of a carbon substrate that was conformally coated with a high Li capacity material. The dimensionality and design for each structure was unique, with each offering different advantages. The addition of an external coating to further increase the stability of high capacity materials was also investigated.

This thesis details a three dimensional model for simulating the operation of two particular configurations of a lithium iron phosphate (LiFePO¬4) battery. Large-scale lithium iron phosphate batteries are becoming increasingly important in a world that demands portable energy that is high in both power and energy density, particularly for hybrid and electric vehicles. Understanding how batteries of this type operate is important for the design, optimization, and control of their performance, safety and durability. While 1D approximations may be sufficient for small scale or single cell batteries, these approximations are limited when scaled up to larger batteries, where significant three dimensional gradients might develop including lithium ion concentration, temperature, current density and voltage gradients. This model is able to account for all of these gradients in three dimensions by coupling an electrochemical model with a thermal model. This coupling shows how electrochemical performance affects temperature distribution and to a lesser extent how temperature affects electrochemical performance. This model is applicable to two battery configurations — spirally wound and prismatically wound. Results generated include temperature influences on current distribution and vice versa, an exploration of various cooling environments’ effects on performance, design optimization of current collector thickness and current collector tab placement, and an analysis of lithium plating risk. / text

The demand for electrical energy storage has increased tremendously in recent years, especially in the applications of portable electronic devices, transportation and renewable energy. The performances of lithium-ion and sodium-ion batteries depend on their electrode materials. In commercial Li-ion batteries with graphite anodes the intercalation potential of lithium in graphite is close to the reversible Li/Li⁺ half-cell potential. The proximity of the potentials can result in unintended electroplating of metallic instead of intercalation of lithium in the graphite anode and frequently leads to internal shorting and overheating, which constitute unacceptable hazards, especially when the batteries are large, as they are in cars and airplanes. Moreover, graphite cannot be readily used as the anode material of Na-ion batteries, because electroplating of metallic sodium on graphite is kinetically favored over sodium intercalation in graphite. This dissertation examines safer Li-ion and Na-ion battery anode materials. / text

Ultrasonic welding is one of the leading technologies for joining multiple, thin sheets of dissimilar materials, such as copper and aluminum, for automotive lithium-ion batteries. The performance of ultrasonic welds, particularly the fatigue life, however, has not been well studied. In this work, a theoretical fatigue life model for ultrasonically welded joints was developed using continuum damage mechanics. In the model, the damage variable was defined as a function of the increase of the joint electrical resistance, resulting in an electrical resistance-based fatigue life prediction model. The fatigue model contains two constants to be determined with experimental data, depending on different fatigue loads and joint properties. As an application, the fatigue life model was validated for Al-Cu lithium-ion battery tab joints. Mechanical fatigue tests were performed under various stress loading conditions for welds made using different welding parameters. It is shown that the developed model can be used to predict the remaining life of the ultrasonically welded battery tab joints for electric and hybrid electric vehicles by monitoring the electrical resistance change. In addition, thermal and electrical fatigue tests were performed for Al-Cu battery tab welds using simulated operating conditions of electrical vehicles. These included temperature cycling between -40 and 90 °C and current cycling of 0 to 10 A. All the tests were conducted on individual weld joints. The results showed that the thermal and electrical loads imposed insignificant effect on the electrical resistance of the battery tab joints. / text

Silicon (Si)-based materials have recently emerged as a promising candidate for anodes in lithium-ion batteries because they exhibit much higher energy-storage capacities than the conventional graphite anode. However, the practical use of Si is hampered by its poor cycleability; during lithiation, Si forms alloys with Li and undergoes significant structural and volume changes, which can cause severe cracking/pulverization and consequent capacity fading arising from the loss of electrical contacts. To overcome these drawbacks, many innovative approaches have been explored with encouraging results; however, many fundamental aspects of the lithiation behavior remain ambiguous. Hence, the focus of this work is to develop a better understanding of the lithiation process at the atomistic scale using quantum mechanical calculations. In addition, based on the improved understanding, we attempt to address the fundamental mechanisms behind the successful approaches to enhance the anode performance. To lay a foundation for the investigation of alloy-type anodes, in Chapter 3, we first examine how lithiation occurs in Si and the formation of crystalline and amorphous LixSi alloys (0 ≤ x ≤ 4); followed by assessing the lithiation-induced changes in the energetics, atomic structure, electronic and mechanical properties, and Li diffusivity. The same approach is then extended to analyze the lithiation behavior of germanium (Ge) and tin (Sn) for developing a generalized understanding on the Group IV alloy-type anodes. Along this comparative study, we notice a few distinguishing features pertain only to Si (or Ge), such as the facile Li diffusion in Ge and facet-dependent lithiation in Si, which are discussed in Chapter 4. Beyond the fundamental research, we also look into factors that may contribute to the improved anode performance, including (i) finetuning of the oxidation effects in Si-rich oxides, [alpha] -SiO [subscript 1/3] (Chapter 5), (ii) maximizing the surface effects through nano-engineered structures (Chapters 6 & 7), and finally (iii) the role of interface in Si-graphene (carbon) composites (Chapter 8).

In this thesis, electrode binders as vital components in the fabrication of composite electrodes for lithium-ion (LIB) and lithium-sulfur batteries (LiSB) have been investigated. Poly(vinylidene difluoride) (PVdF) was studied as binder for sulfur-carbon positive electrodes by a combination of galvanostatic cycling and nitrogen absorption. Poor binder swelling in the electrolyte and pore blocking in the porous carbon were identified as origins of low discharge capacity, rendering PVdF-based binders an unsuitable choice for LiSBs. More promising candidates are blends of poly(ethylene oxide) (PEO) and poly(N-vinylpyrrolidone) (PVP). It was found that these polymers interact with soluble lithium polysulfide intermediates generated during the cell reaction. They can increase the discharge capacity, while simultaneously improving the capacity retention and reducing the self-discharge of the LiSB. In conclusion, these binders improve the local electrolyte environment at the electrode interface. Graphite electrodes for LIBs are rendered considerably more stable in ‘aggressive’ electrolytes (a propylene carbonate rich formulation and an ether-based electrolyte) with the poorly swellable binders poly(sodium acrylate) (PAA-Na) and carboxymethyl cellulose sodium salt (CMC-Na). The higher interfacial impedance seen for the conventional PVdF binder suggests a protective polymer layer on the particles. By reducing the binder content, it was found that PAA-Na has a stronger affinity towards electrode components with high surface areas, which is attributed to a flexible polymer backbone and a higher density of functional groups. Lastly, a graphite electrode was combined with a sulfur electrode to yield a balanced graphite-sulfur cell. Due to a more stable electrode-electrolyte interface the self-discharge of this cell could be reduced and the cycle life was extended significantly. This example demonstrates the possible benefits of replacing the lithium metal negative electrode with an alternative electrode material.

Lithium-ion (Li-ion) batteries are critically important for portable electronics, electric vehicles, and grid-level energy storage. The development of next-generation Li-ion batteries requires high-capacity electrodes with a long cycle life. However, the high capacity of Li storage is usually accompanied by large volume changes, dramatic morphological evolution, and mechanical failures in the electrodes during charge and discharge cycling. To understand the degradation of electrodes and resulting loss of capacity, this thesis aims to develop mechanistic-based models for predicting the chemo-mechanical processes of lithiation and delithiation in high-capacity electrode materials. To this end, we develop both continuum and atomistic models that simulate mass transport, interface reaction, phase and microstructural evolution, stress generation and damage accumulation through crack or void formation in the electrodes. The modeling studies are tightly coupled with in-situ transmission electron microscopy (TEM) experiments to gain unprecedented mechanistic insights into electrochemically-driven structural evolution and damage processes in high-capacity electrodes. Our models are successfully applied to the study of the two-phase lithiation and associated stress generation in both crystalline and amorphous silicon anodes, which have the highest known theoretical charge capacity, as well as the lithiation/sodiation-induced structural changes and mechanical failures in silicon-based multilayer electrodes. The modeling studies have uncovered unexpected electrochemical reaction mechanisms and revealed novel failure modes in silicon-based nanostructured anodes. Our modeling research provides insights into how to mitigate electrode degradation and enhance capacity retention in Li-ion batteries. More broadly, our work has implications for the design of nanostructured electrodes in next-generation energy storage systems.