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.

Polymer nanocomposites are used in a variety of applications due to their good mechanical properties. Specifically, better performance of lithium ion batteries and thermal interface material can be obtained by using conductive materials and polymer composites. In the case of lithium ion batteries, electrochemical properties of batteries can be improved by adding conductive additives and conducting polymer into the cathode. Several samples, to which different conductive additives and conducting polymer were added, were prepared and their electrical resistance and discharge capacity measured. In the thermal interface material case, also, thermal properties can be enhanced by polymer nanocomposites. In order to confirm the thermal conductivity enhancement, samples were synthesized using different filler, polymer and methods, and their thermal conductivity measured. The influence of polymer nanocomposites and results are discussed and future plan are presented. In addition, reasons of thermal conductivity changing in each case are discussed.

Charge equalization is a major issue in the service of batteries since they are frequently connected in series to obtain higher output voltage levels for most applications. With series connection, imbalance may happen to the operating batteries during either charging or discharging periods. The imbalance among batteries concerns the operating efficiency and the battery lifetime. The main object of this dissertation is to solve the problem of charge inequality. The importance of charge equalization is first addressed. The problem is demonstrated by experiments of charging/discharging processes. Then, the techniques of battery charging and charge equalization are reviewed. To improve charge equalization, a dynamic balance charging scheme is developed on the basis of buck-boost conversion. The balance charging scheme can be realized by two configurations, that is, ¡§forward allotting¡¨ or ¡§backward allotting¡¨ configurations. The circuits are composed of several duplicated subcircuits and operated by digital control kernel, therefore, they are easy to be applied on battery sets with different numbers of batteries. By dynamically re-allocating the energy drawing from satiated batteries and allotted to hungry ones, the series-connected batteries can reach balance state more efficiently. The balance charging circuits can be employed during off-line or even discharging. However, on observing that the output voltage will vary in a big range when the battery set is discharged, the charge equalization can be integrated with voltage regulation on the output. Evolve from this idea, a balance discharging circuit¡@topology based on multi-winding transformer is proposed. The experiments in this dissertation are carried out on lead-acid batteries, therefore, the reactions and characteristics of lead-acid batteries are discussed. However, the proposed circuits are not restricted to be applied on lead-acid batteries only. Experimental results confirm the theoretical analyses and manifest the effectiveness of the designed circuits.

Thesis (M.S. in Electrical Engineering)--Naval Postgraduate School, December 2009. / Thesis Advisor(s): Julian, Alexander L. Second Reader: Crisiti, Roberto. "December 2009." Description based on title screen as viewed on January 28, 2010. Author(s) subject terms: Pulsed power, charger, buck converter, field programmable gate array (FPGA), lithium-ion batteries. Includes bibliographical references (p. 77-79). Also available in print.

Thesis (Ph. D.)--University of Hong Kong, 2002. / Includes bibliographical references.

Growing environmental concerns and increasing demand for energy have stimulated extensive interest in electrical energy storage. Li-air batteries are appealing in this regard as they offer much higher energy density than the current Li-ion batteries, but the nonaqueous Li-air batteries suffer from poor cycle life arising from electrolyte decomposition and clogging of the air electrode by insoluble discharge products. Interestingly, hybrid Li-air batteries in which a solid electrolyte separates the lithium-metal anode in an aprotic electrolyte from the air electrode in an aqueous catholyte could overcome these problems. Lots of efforts have been made on developing efficient bifunctional catalysts to lower the overpotential and improve the stability of hybrid Li-air batteries, but the cycle life is still limited. This dissertation focuses on the development of advanced cell configurations and high-performance catalysts for hybrid Li-air batteries. First, a buffer catholyte solution with a moderate pH, based on phosphoric acid and supporting salts, has been developed to keep the solid electrolyte stable and reduce the internal resistance and overpotential. With a high operating voltage and the utilization of all the three protons of phosphoric acid, the buffer catholyte enables a Li-air cell with high energy density. Further increase in power density has been realized by increasing the solid-electrolyte conductivity and operating temperature to 40 °C. The biggest challenge with Li-air cells is the large overpotentials associated with the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Noble-metal-free NiCo₂O₄ nanoflakes directly grown onto a nickel foam (NCONF@Ni) has been found to exhibit high OER activity that is comparable to that of the expensive, noble-metal IrO₂ catalyst. Furthermore, a novel 3-D O- and N-doped carbon nanoweb (ON-CNW) has been developed as an inexpensive, metal-free catalyst for ORR. With a hybrid Li-air cell, the ON-CNW exhibits performance close to that of commercial Pt/C. In addition, a novel hybrid Li-air cell configuration with decoupled ORR and OER electrodes has been developed. The hybrid Li-air cell with decoupled ORR and OER electrodes eliminates the degradation of ORR catalysts and carbon support in the highly oxidizing charge process and leads to high efficiency with good cycle life. / text

Li-ion batteries are widely used in electronics and automotives. Despite their success, improvements in cost, safety, cycle life, and energy density are necessary. One way to enhance the energy density is to find advanced cathodes such as Li-rich layered oxides, which are similar to the commonly layered oxide cathodes (e.g., LiCoO2), except there are additional Li ions in the transition-metal layer, due to their higher charge-storage capacity. Another way of advancing is to design new battery chemistries, such as those involving multivalent-ion systems (e.g., Mg2+ and Zn2+) as they could offer higher charge-storage capacities and/or cost advantages. Li-rich layered oxides have a complex first charge-discharge cycle, which affects their other electrochemical properties. Ru doping was expected to improve the performance of Li-rich layered oxides due to its electroactivity and overlap of the Ru4+/5+:4d band with the O2-:2p band, but it unexpectedly decreased the capacity due to the reduction in oxygen loss behavior. Preliminary evidence points to the formation of Ru-Ru dimers, which raises the Ru4+/5+:4d band, as the cause of this behavior. Li-rich layered oxides suffer from declining operating voltage during cycling, and it is a huge challenge to employ them in practical cells. Raising the Ni oxidation state was found to reduce the voltage decay and improve the cyclability; however, it also decreased the discharge capacity. Increasing the Ni oxidation state minimized the formation of Mn3+ ions during discharge and Mn dissolution, which led to the improvements in voltage decay and cyclability. Extraction of lithium from spinel oxides such as LiMn2O4 with acid was found to follow a Mn3+ disproportionation mechanism and depend on the Mn3+ content. Other common dopants like Cr3+, Fe3+, Co3+, or Ni2+/3+ did not disproportionate, and no ion-exchange of Li+ with H+ occurred in the tetrahedral sites of the spinel oxides. Extraction with acid of Mg and Zn from spinel oxides, such as MgMn2O4 and ZnMn2O4, were also found to follow the same mechanism as Li-spinels. The Mg-spinels, however, do experience ion exchange when Mg ions are in the octahedral sites. Chemical extraction of Mg or Zn with an oxidizing agent NO2BF4 in acetonitrile medium, however, failed due to the electrostatic repulsion felt by the migrating divalent ions. In contrast, extraction with acid was successful as Mn dissolution from the lattice opened up favorable pathways for extraction. / text

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.

This thesis investigates the mechanical behavior of electrodes in Li-ion batteries. Each electrode in a Li-ion battery consists of host atoms and guest atoms (Li atoms). The host atoms form a framework, into which Li atoms are inserted via chemical reactions. During charge and discharge, the amount of Li in the electrode varies substantially, and the host framework deforms. The deformation induces in an electrode a field of stress, which may lead to fracture or morphological change. Such mechanical degradation over lithiation cycles can cause the capacity to fade substantially in a commercial battery. We study fracture of elastic electrodes caused by fast charging using a combination of diffusion kinetics and fracture mechanics. A theory is outlined to investigate how material properties, electrode particle size, and charging rate affect fracture of electrodes in Li-ion batteries. We model an inelastic host of Li by considering diffusion, elastic-plastic deformation, and fracture. The model shows that fracture is averted for a small and soft host—an inelastic host of a small feature size and low yield strength. We present a model of concurrent reaction and plasticity during lithiation of crystalline silicon electrodes. It accounts for observed lithiated silicon of anisotropic morphologies. We further explore the microscopic deformation mechanism of lithiated silicon based on first-principles calculations. We attribute to the microscopic mechanism of large plastic deformation to continuous Li-assisted breaking and reforming of Si-Si bonds. In addition, we model the evolution of the biaxial stress in an amorphous Si thin film electrode during lithiation cycle. We find that both the atomic insertion driven by the chemomechanical load and plasticity driven by the mechanical load contribute to reactive flow of lithiated silicon. In such concurrent process, the lithiation reaction promotes plastic deformation by lowering the stress needed to flow. Li-ion battery is an emerging field that couples electrochemistry and mechanics. This thesis aims to understand the deformation mechanism, stresses and fracture associated with the lithiation reaction in Li-ion batteries, and hopes to provide insight on the generic phenomenon that involves interactive chemical reactions and mechanics. / Engineering and Applied Sciences