<p>This thesis aims at formulating and validating models for electrochemical energy storage devices. More specifically, the devices under consideration are lithium ion batteries and polymer electrolyte fuel cells.</p><p>A model is formulated to describe an experimental cell setup consisting of a Li_xNi_0.8Co_0.15Al_0.05O₂ composite porous electrode with three porous separators and a reference electrode between a current collector and a pure Li planar electrode. The purpose of the study being the identification of possible degradation mechanisms in the cell, the model contains contact resistances between the electronic conductor and the intercalation particles of the porous electrode and between the current collector and the porous electrode. On the basis of this model formulation, an analytical solution is derived for the impedances between each pair of electrodes in the cell. The impedance formulation is used to analyse experimental data obtained for fresh and aged Li_xNi_0.8Co_0.15Al_0.05O₂ composite porous electrodes. Ageing scenarios are formulated based on experimental observations and related published electrochemical and material characterisation studies. A hybrid genetic optimisation technique is used to simultaneously fit the model to the impedance spectra of the fresh, and subsequently also to the aged, electrode at three states of charge. The parameter fitting results in good representations of the experimental impedance spectra by the fitted ones, with the fitted parameter values comparing well to literature values and supporting the assumed ageing scenario.</p><p>Furthermore, a steady state model for a polymer electrolyte fuel cell is studied under idealised conditions. The cell is assumed to be fed with reactant gases at sufficiently high stoichiometric rates to ensure uniform conditions everywhere in the flow fields such that only the physical phenomena in the porous backings, the porous electrodes and the polymer electrolyte membrane need to be considered. Emphasis is put on how spatially resolved porous electrodes and nonequilibrium water transport across the interface between the gas phase and the ionic conductor affect the model results for the performance of the cell. The future use of the model in higher dimensions and necessary steps towards its validation are briefly discussed.</p>

This study investigates the economic benefits of installing a lithium-ion battery storage (lithium iron phosphate, LFP and lithium titanate, LTO) at an electric bus fast charging station.  It is conducted on a potential electric bus system in the Swedish city Västerås, and based on the existing bus schedules and routes as well as the local distribution system. The size of the energy storage as well as the maximum power outtake from the grid is optimized in order to minimize the total annual cost of the connection. The assessment of the distribution system shows that implementing an electric bus system based on opportunity charging in Västerås does not cause over-capacity in the 10 kV grid during normal feeding mode. However, grid reinforcements might become necessary to guarantee potential backup feeding modes. Batteries are not a cost effective option to decrease grid owner investments in new transformers. However, battery energy storage have the possibility to decrease the annual cost of connecting a fast charging station to the low-voltage grid. The main advantage of the storage system is to decrease the fees to the grid owner. Of the studied batteries, LTO is the most cost effective solution because of its larger possible depth-of-discharge for a given cycle life. The most important characteristics, that determine if a fast charging station could benefit economically from an energy storage, is the bus frequency. The longer the time in between buses and the higher the power demand, the more advantageous is the energy storage.

We are today experiencing an increasing demand for high energy density storage devices like the lithium-ion battery for applications in portable electronic devices, electric vehicles (EV) and as interim storage for renewable energy. High capacity retention and long cycle life are prerequisites, particularly for the EV market. The key for a long cycle life is the formation of a stable solid-electrolyte interphase (SEI) layer on the surface of the negative electrode, which typically forms on the first cycles due to decomposition reactions at the electrode-electrolyte interface. More control over the surface layer can be gained when the layer is generated prior to the battery operation. Such a layer can be tailored more easily and can reduce the loss of lithium inventory considerably. In this context, water-soluble electrode binders, e.g. sodium carboxymethyl cellulose (CMC-Na) and poly(acrylic acid) (PAA), have proven themselves exceptionally useful. Since the binder is a standard component in composite electrodes anyway, its integration into the electrode fabrication process is easily accomplished. This thesis work investigates the parameters that govern binder distribution in elec-trode coatings, control the stability and electrochemical performance of the elec-trode and that determine the composition of the surface layer. Several commonly used electrode materials (graphite, silicon and lithium titanate) have been applied in order to study the impact of the binder on the electrode morphology and the differ-ent electrode-electrolyte interfaces. The results are correlated with the electrochemi-cal performance and with the SEI composition obtained by in-house and synchro-tron-based photoelectron spectroscopy (PES). The results demonstrate that the poor swellability of these water-soluble binders leads to a protection of the active material, given that the surface coverage is high and the binder evenly distributed. Although on the laboratory scale electrode formu-lations with a high binder content are common, they have little practical use in commercial devices due to the high content of inactive material. As the binder con-tent is decreased, complete surface coverage is more difficult to achieve and the binder distribution is more strongly coupled to the particle-binder interactions during the preparation process. Moreover, it is demonstrated in this thesis how these inter-actions are related to the surface area of the electrode components applied, the surface composition and the electrochemistry of the electrode. As a result of the smaller binder contents the benefits provided by CMC-Na and PAA at the electrode surface are compromised and the performance differs less distinctly from electrodes fabricated with the conventional binder, i.e. poly(vinylidene difluoride) (PVdF). Composites of alloying and conversion materials, on the other hand, typically em-ploy binders in larger amounts. Despite the frequently noted resiliency to volume expansion, which is also a positive side effect of the poor swellability of the binder in the electrolyte, the protection of the surface and the formation of a more stable interface are the major cause for the improved electrochemical behaviour, com-pared to electrodes employing PVdF binders.

Silicon-based electrodes are the most promising negative electrodes for the next generation high capacity lithium ion batteries (LIB) as silicon provides a theoretical capacity of 3579 mAh g-1, more than 10 times higher than that of the state-of-the-art graphite negative electrodes. However, silicon-based electrodes suffer from poor cycle life due to large volume expansion and contraction during lithiation/delithiation. In order to improve the electrochemical performance a number of strategies have been employed, such as dispersion of silicon in active/inactive matrixes, devising of novel nanostructures, and various coatings for protection. Amongst these strategies, silicon-carbon coating based composites are one of the most promising because carbon coating is comparatively flexible, easy to obtain, and scalable with various industrial processes. Low cost and renewable lignin, which constitutes up to 30% dry mass of the organic carbon on earth, is widely available from paper and pulp mills which produce lignin in excess of 50 million tons annually worldwide. It is a natural bio-polymer with high carbon content and highly interconnected aromatic network existing as a structural adhesive found in plants. Generally burnt for energy on site, lignin is gradually finding its way into high value-added products such as precursor for carbon fibers, active material in negative electrodes, and raw material for supercapacitors. This dissertation focuses on high performance silicon-based negative electrodes utilizing lignin as the carbon precursor for conductive additive, binder, and carbon coating. To my knowledge this is one of the first works attempting to utilize and summarize the performance of lignin in silicon-based negative electrodes. The first part of the dissertation shows that silicon-lignin composites treated at 800 ºC displayed good capacity and cycling performance. The second part goes to generalize the effect of temperature on silicon-lignin composites and shows that a low temperature treatment granted an electrode with superior performance and cycling properties owing to the preservation of polymeric properties of lignin. The final part of the dissertation discusses the current research trends in SiOx based negative electrodes and extends lignin to that field. This dissertation will, hopefully, provide knowledge and insight for fellow researchers wishing to utilize lignin or other renewable resources in devising advanced battery electrodes.

We consider the prediction algorithm and performance evaluation for prognostics and health management (PHM) problems, especially the prediction of remaining useful life (RUL) for the milling machine cutter and lithium ‐ ion battery. We modeled battery as a voltage source and internal resisters. By analyzing voltage change trend during discharge, we made the prediction of battery remain discharge time in one discharge cycle. By analyzing internal resistance change trend during multiple cycles, we were able to predict the battery remaining useful time during its life time. We showed that the battery rest profile is correlated with the RUL. Numerical results using the realistic battery aging data from NASA prognostics data repository yielded satisfactory performance for battery prognosis as measured by certain performance metrics. We built a battery test platform and simulated more usage pattern and verified the prediction algorithm. Prognostic performance metrics were used to compare different algorithms.

Thesis advisor: Dunwei Wang / In this work, we present our findings regarding the low-temperature, solid-state conversion of Cu₂S nanowires to Cu₂S/Cu₅FeS₄ rod-in-tube structures, Cu₂S/ZnS segmented nanowires, and a full conversion of Cu₂S nanowires to ZnS nanowires. These conversion reactions occur at temperatures as low as 105 degrees Celsius, a much lower temperature than those required for reported solid-state reactions. The key feature of the Cu₂S nanowires that enables such low conversion temperatures is the high ionic diffusivity of the Cu⁺ within a stable S sublattice. The second portion of this work will focus on the oxide-stabilization and utilization of TiSi₂ nanonets as a lithium-ion battery anode. This nanostructure, first synthesized in our lab, was previously demonstrated to possess a lithium storage capacity when cycled against a metallic Li electrode. However, with subsequent lithiation and delithiation cycles, the TiSi₂ nanonet structure was found to be unstable. By allowing a thin oxide layer to form on the surface of the nanonet, we were able to improve the capacity retention of the nanonets in a lithium-ion half-cell; 89.8% of the capacity of the oxide-coated TiSi₂ was retained after 300 cycles compared to 62.3% of the capacity of as-synthesized TiSi₂ nanonets after 300 cycles. The layered structure of C49 TiSi₂ exhibited in the nanonets allows for a specific capacity greater than 700 mAh g(-1), and the high electrical conductivity of the material in conjunction with the layered structure confer the ability to cycle the anode at rates of up to 6C, i.e., 10 minute charge and discharge cycles, while still maintaining more than 75% of the capacity at 1C, i.e., 1 hour charge and discharge cycles. / Thesis (MS) — Boston College, 2013. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

Silicon (in the form of lithium silicides) has almost ten times the theoretical charge storage capacity of graphite, the anode material used in most commercially-available lithium-ion batteries. Replacing graphite with silicon therefore promises a substantial improvement over the state-of-the-art in electrochemical energy storage. However, it has proved difficult to realise this high theoretical capacity in a practical electrochemical cell and maintain it over repeated charge-discharge cycles. This dissertation presents experimental work probing the changes in local structure occurring during the electrochemical reactions of lithium with silicon, using neutron total scattering and nuclear magnetic resonance, together with novel processing methodologies for analysing the resulting data, in the hope of suggesting ways of improving the performance of silicon-based lithium-ion batteries. Neutron total scattering patterns were obtained from silicon-based anode materials extracted from cells at various states of charge. These samples were composed of a heterogeneous mixture of amorphous, crystalline and disordered crystalline materials. Reverse Monte Carlo is a technique for obtaining structural information from experimental data (particularly total scattering patterns) from amorphous and disordered crystalline materials. However, previously existing Reverse Monte Carlo software could only handle homogeneous materials. Therefore, the RMCprofile software package was extended to handle data from heterogeneous samples. The improved RMCprofile was applied to the aforementioned total scattering patterns, but the much stronger scattering from the other components (themselves not well-characterised) swamped that from the lithium silicide. Future work should attempt to reduce the scattering from the inactive components, particularly the hard-to-model incoherent scattering. NMR data were acquired in situ from silicon-nanowire-based lithium-ion batteries during repeated charge-discharge cycles, achieving much better electrochemical performance than had been seen in previous in situ experiments with silicon. Owing to the large quantities of data obtained, an automated, model-free dimensionality reduction technique was needed. The NMR data were processed using principal component analysis and a variant of non-negative matrix factorisation. With both of these methods, one of the components was found to be associated with high voltages vs. ${Li \vert{} Li^{+}}$ (i.e. a fully discharged anode). This region has seen very little interest by comparison with the low voltage (high levels of lithiation) region of the charge-discharge cycle, so this discovery suggests a new avenue for future research.

Current demands on lithium ion battery (LIB) technology include high capacity retention over a life time of many charge and discharge cycles. Maximizing battery longevity is still a major challenge partly due to electrode degradation as a function of repeated cycling. The intercalation of lithium ions into an active material causes the development of stress and strain in active electrode materials which can result in fracture and shifting that can in turn lead to capacity fade and eventual cell failure. The processes leading to active material degradation in cycling LIBs has been studied using a combination of acoustic emission (AE) and in situ X-ray diffraction (XRD) techniques. Safe, low cost custom electrochemical cells were designed and developed for use in battery AE and XRD experiments. These tools were used to monitor the time of material fracture through AE and link these events to lattice strain and phase composition as determined by XRD. Both anode and cathode materials were studied with an emphasis on graphite, silicon, and Li(Mn1.5Ni0.5)O4, and tin. A thermal analogy model for lithiation/delithiation induced fracture of spherical particles capable of predicting when AE should be detected in a cell containing a composite silicon electrode. The results of this work were used to develop an understanding of when and how active materials are degrading as well as to suggest methods of improving their performance and operational longevity.

Within the framework of this thesis, three innovative electrochemical devices have been studied. A part of the work is devoted to an already existing device, laminates which are debonded by the application of a voltage. This type of material can potentially be used in a wide range of applications, including adhesive joints in vehicles to both reduce the total weight and to simplify the disassembly after end-of-life, enabling an inexpensive recycling process. Although already a functioning device, the development and tailoring of this process was slowed by a lack of knowledge concerning the actual electrochemical processes responsible for the debonding. The laminate studied consisted of an epoxy adhesive, mixed with an ionic liquid, bonding two aluminium foils. The results showed that the electrochemical reaction taking place at the releasing anode interface caused a very large increase in potential during galvanostatic polarization. Scanning electron microscopy images showed reaction products growing out from the electrode surface into the adhesive. These reaction products were believed to cause the debonding through swelling of the anodic interface so rupturing the adhesive bond. The other part of the work in this thesis was aimed at innovative lithium ion (Li‑ion) battery concepts. Commercial Li-ion batteries are two-dimensional thin film constructions utilized in most often mechanically rigid products. Two routes were followed in this thesis. In the first, the aim was flexible batteries that could be used in applications such as bendable reading devices. For this purpose, nano-fibrillated cellulose was used as binder material to make flexible battery components. This was achieved through a water-based filtration process, creating flexible and strong papers. These paper-based battery components showed good mechanical properties as well as good rate capabilities during cycling. The drawback using this method was relatively low coulombic efficiencies believed to originate from side-reactions caused by water remnants in the cellulose structure. The second Li-ion battery route comprised an electrochemical process to coat carbon fibers, shown to perform well as negative electrode in Li-ion batteries, from a monomer solution. The resulting polymer coatings were ~500 nm thick and contained lithium ions. This process could be controlled by mainly salt content in the monomer solution and polarization time, yielding thin and apparently pin-hole free coatings. By utilizing the carbon fiber/polymer composite as integrated electrode and electrolyte, a variety of battery designs could possibly be created, such as three-dimensional batteries and structural batteries. / <p>QC 20130403</p>

The exploration of functional materials to store renewable, clean, and efficient energies for electric vehicles (EVs) has become one of the most popular topics in both material chemistry and electrochemistry. Rechargeable lithium batteries and fuel cells are considered as the most promising candidates, but they are both facing some challenges before the practical applications. For example, the low discharge capacity and energy density of the current lithium ion battery cannot provide EVs expected drive range to compete with internal combustion engined vehicles. As for fuel cells, the rapid and safe storage of H2 gas is one of the main obstacles hindering its application. In this thesis, novel mesoporous/nano functional materials that served as cathodes for lithium sulfur battery and lithium ion battery were studied. Ternary lithium transition metal nitrides were also synthesized and examined as potential on-board hydrogen storage materials for EVs. Highly ordered mesoporous carbon (BMC-1) was prepared via the evaporation-induced self-assembly strategy, using soluble phenolic resin and Tetraethoxysilane (TEOS) as precursors and triblock copolymer (ethylene oxide)106(propylene oxide)70(ethylene oxide)106 (F127) as the template. This carbon features a unique bimodal structure (2.0 nm and 5.6 nm), coupled with high specific area (2300 m2/g) and large pore volume (2.0 cm3/g). The BMC-1/S nanocomposites derived from this carbon with different sulfur content exhibit high reversible discharge capacities. For example, the initial capacity of the cathode with 50 wt% of sulfur was 995 mAh/g and remains at 550 mAh/g after 100 cycles at a high current density of 1670 mA/g (1C). The good performance of the BMC-1C/S cathodes is attributed to the bimodal structure of the carbon, and the large number of small mesopores that interconnect the isolated cylindrical pores (large pores). This unique structure facilitates the transfer of polysulfide anions and lithium ions through the large pores. Therefore, high capacity was obtained even at very high current rates. Small mesopores created during the preparation served as containers and confined polysulfide species at the cathode. The cycling stability was further improved by incorporating a small amount of porous silica additive in the cathodes. The main disadvantage of the BMC-1 framework is that it is difficult to incorporate more than 60 wt% sulfur in the BMC-1/S cathodes due to the micron-sized particles of the carbon. Two approaches were employed to solve this problem. First, the pore volume of the BMC-1 was enlarged by using pore expanders. Second, the particle size of BMC-1 was reduced by using a hard template of silica. Both of these two methods had significant influence on improving the performance of the carbon/sulfur cathodes, especially the latter. The obtained spherical BMC-1 nanoparticles (S-BMC) with uniform particle size of 300 nm exhibited one of the highest inner pore volumes for mesoporous carbon nanoparticles of 2.32 cm3/g and also one of the highest surface areas of 2445 m2/g with a bimodal pore size distribution of large and small mesopores of 6 nm and 3.1 nm. As much as 70 wt% sulfur was incorporated into the S-BMC/S nanocomposites. The corresponding electrodes showed a high initial discharge capacity up to 1200 mAh/g and 730 mAh/g after 100 cycles at a high current rate 1C (1675 mA/g). The stability of the cells could be further improved by either removal of the sulfur on the external surface of spherical particles or functionalization of the C/S composites via a simple TEOS induced SiOx coating process. In addition, the F-BMC/S cathodes prepared with mesoporous carbon nanofibers displayed similar performance as the S-BMC/S. These results indicate the importance of particle size control of mesoporous carbons on electrochemical properties of the Li-S cells. By employing the order mesoporous C/SiO2 framework, Li2CoSiO4/C nanocomposites were synthesized via a facile hydrothermal method. The morphology and particle size of the composites could be tailored by simply adjusting the concentrations of the base LiOH. By increasing the ratio of LiOH:SiO2:CoCl2 in the precursors, the particle size decreased at first and then went up. When the molar ratio is equal to 8:1:1, uniform spheres with a mean diameter of 300-400 nm were obtained, among which hollow and core shell structures were observed. The primary reaction mechanism was discussed, where the higher concentration of OH- favored the formation of Li2SiO3 but hindered the subsequent conversion to Li2CoSiO4. According to the elemental maps and TGA of the Li2CoSiO4/C, approximately 2 wt% of nanoscale carbon was distributed on/in the Li2CoSiO4, due to the collapse of the highly ordered porous structure of MCS. These carbons played a significant role in improving the electrochemical performance of the electrode. Without any ball-mill or carbon wiring treatments, the Li2CoSiO4/C-8 exhibited an initial discharge capacity of 162 mAh/g, much higher than that of the sample synthesized with fume silica under similar conditions and a subsequent hand-mixing of Ketjen black. Finally, lithium transition metal nitrides Li7VN4 and Li7MnN4 were prepared by high temperature solid-state reactions. These two compounds were attempted as candidates for hydrogen storage both by density functional theory (DFT) calculations and experiments. The results show that Li7VN4 did not absorb hydrogen under our experimental conditions, and Li7MnN4 was observed to absorb 7 hydrogen atoms through the formation of LiH, Mn4N, and ammonia gas. While these results for Li7VN4 and Li7MnN4 differ in detail, they are in overall qualitative agreement with our theoretical work, which strongly suggests that both compounds are unlikely to form quaternary hydrides.