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).

Electric and hybrid electric vehicles are gaining momentum as a sustainable alternative to conventional combustion based transportation. The operating temperature of the vehicle will vary significantly over the vehicle lifetime and this variance in operating temperature will strongly impact the performance, driving range, and durability of batteries used in the vehicles. In the first part of this thesis, an experimental facility is developed to accurately quantify the effects of battery operating temperature on discharge characteristics through precise control of the battery operating temperatures, utilizing a water-ethylene glycol solution in a constant temperature thermal bath. A prismatic 20Ah LiFEPO4 battery from A123 is tested using the developed method, and temperature measurements on the battery throughout discharge show a maximum variation of 0.3°C temporally and 0.4°C spatially at a 3C discharge rate, in contrast to 13.1°C temperature change temporally and 4.3°C spatially when using the conventional air convection temperature control method under the same test conditions. A comparison of battery discharge curves using the two methods show that the reduction in spatial and temporal temperature change in the battery has a large effect on the battery discharge characteristics. The developed method of battery temperature control yields more accurate battery discharge characterization due to both the elimination of state-of-charge drift caused by spatial variations in battery temperature, and inaccurate discharge characteristics due to battery heat up at various discharge and ambient conditions. Battery discharge characterization performed using the developed method of temperature control exhibits a reduction in battery capacity of 95% when the operating temperature is decreased from 20°C to -10°C at 3C discharge rate. A reduction of 35% in battery capacity is observed when for the same temperature decrease at a 0.2C discharge rate. The observed effect of operating temperature on the capacity of the tested battery highlights the importance of an effective thermal management system, the design of which requires accurate knowledge of the heat generation characteristics of the battery under various discharge rates and operating temperatures. In the second part of this thesis, a calorimeter capable of measuring the heat generation rates of a prismatic battery is developed and verified by using a controllable electric heater. The heat generation rate of a prismatic A123 LiFePO4 battery is measured for discharge rates ranging from 0.25C to 3C and operating temperature ranging from -10°C to 40°C. Results show that the heat generation rates of Lithium ion batteries are greatly affected by both battery operating temperature and discharge rate. At low rates of discharge the heat generation is not significant, even becoming endothermic at the battery operating temperatures of 30°C and 40°C. Heat of mixing is observed to be a non-negligible component of total heat generation at discharge rates as low as 0.25C for all tested battery operating temperatures. A double plateau in battery discharge curve is observed for operating temperatures of 30°C and 40°C. The developed experimental facility can be used for the measurement of heat generation for any prismatic battery, regardless of chemistries. The characterization of heat generated by the battery under various discharge rates and operating temperatures can be used to verify the accuracy of battery heat generation models currently used, and for the design of an effective thermal management system for electric and hybrid electric vehicles in the automotive industry.

Lithium-rich transition metal oxides are candidates for the next-generation lithium-ion battery positive electrode materials. They have a much higher first charge and low-rate cycling capacity compared to non-lithium rich transition metal oxides. In this thesis, the preparation of spherical and dense transition metal oxide was studied. The morphology and tap-density of the hydroxide precursor was found to be dependent on the coprecipitation reaction pH. The coprecipitation reaction in the presence of aqueous ammonia was studied by analyzing the relevant chemical equilibria. The electrochemistry of lithium-rich oxides was studied as a function of particle size. The apparent oxygen diffusion coefficients were estimated using the Atlung graph method and were determined to be several orders of magnitude lower than normal lithium deintercalation. Isothermal mass calorimetry measurements showed evidence of a local Jahn-Teller distortion in the MnO6 units during discharge. Other studies of the lithium-rich oxides were also carried out.

Silicon is a promising negative electrode material for lithium-ion (Li-ion) batteries, with volumetric and gravimetric capacities much higher than those in current commercial batteries. Implementation of Si as a negative electrode is halted, however, by a large irreversible capacity and declining reversible capacity over cycle life. These problems are linked to the large volume expansion that Si undergoes when reacted with lithium, and overcoming them is the focus of this thesis. To overcome this expansion, in the first part titanium silicides were proposed to buffer the volume expansion problem as Ti does not react with Li and is robust. A pure phase of the targeted TiSi and TiSi2 was not achieved, however one product mixture containing TiSi2 and Ti5Si3 was cycled against Li at C/20. A capacity of 715 mAh g-1 was achieved, however rapid capacity fade occurred over the first 10 cycles. The second part of the thesis focused on heterostructured Si-Ge and Ge-Si core- shell nanowires. The morphology of the nanowires allows for better accommodation of strain due to lithiation, and Ge functions as an active matrix, as it can store Li in a similar manner as Si. The specific capacities of the nanowires were good at 1346 mAh g-1 and 1276 mAh g-1, however after 50 cycles the Si-Ge nanowires had a capacity retention of 72.4 % and the Ge-Si retained 62.4 %. The diffusion coefficient of Li was determined from GITT and EIS to be within the range of 10-16 to 10-13 cm2s-1 and was slightly lower than other reported values, attributed to the dense structure of the nanowires slowing diffusion.

京都大学 / 新制・課程博士 / 博士(エネルギー科学) / 甲第24001号 / エネ博第437号 / 新制||エネ||82(附属図書館) / 京都大学大学院エネルギー科学研究科エネルギー基礎科学専攻 / (主査)准教授 高井 茂臣, 教授 萩原 理加, 教授 佐川 尚 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DFAM

abstract: Tin (Sn) has a high-specific capacity (993 mAhg-1) as an anode material for Li-ion batteries. To overcome the poor cycling performance issue caused by its large volume expansion and pulverization during the charging and discharging process, many researchers put efforts into it. Most of the strategies are through nanostructured material design and introducing conductive polymer binders that serve as matrix of the active material in anode. This thesis aims for developing a novel method for preparing the anode to improve the capacity retention rate. This would require the anode to have high electrical conductivity, high ionic conductivity, and good mechanical properties, especially elasticity. Here the incorporation of a conducting polymer and a conductive hydrogel in Sn-based anodes using a one-step electrochemical deposition via a 3-electrode cell method is reported: the Sn particles and conductive component can be electrochemically synthesized and simultaneously deposited into a hybrid thin film onto the working electrode directly forming the anode. A well-defined three dimensional network structure consisting of Sn nanoparticles coated by conducting polymers is achieved. Such a conductive polymer-hydrogel network has multiple advantageous features: meshporous polymeric structure can offer the pathway for lithium ion transfer between the anode and electrolyte; the continuous electrically conductive polypyrrole network, with the electrostatic interaction with elastic, porous hydrogel, poly (2-acrylamido-2-methyl-1-propanesulfonic acid-co-acrylonitrile) (PAMPS) as both the crosslinker and doping anion for polypyrrole (PPy) can decrease the volume expansion by creating porous scaffold and softening the system itself. Furthermore, by increasing the amount of PAMPS and creating an interval can improve the cycling performance, resulting in improved capacity retention about 80% after 20 cycles, compared with only 54% of that of the control sample without PAMPS. The cycle is performed under current of 0.1 C. / Dissertation/Thesis / Masters Thesis Materials Science and Engineering 2015

Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第15672号 / 工博第3330号 / 新制||工||1502(附属図書館) / 28209 / 京都大学大学院工学研究科物質エネルギー化学専攻 / (主査)教授 安部 武志, 教授 垣内 隆, 教授 井上 正志 / 学位規則第4条第1項該当