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Thesis (Ph.D. (Physics)) -- University of Limpopo, 2019 / LiMn2O4 spinel (LMO) is a promising cathode material for secondary lithium-ion batteries which, despite its high average voltage of lithium intercalation, suffers crystal symmetry lowering due to the Jahn-Teller active six-fold Mn3+ cations. Although Ni has been proposed as a suitable substitutional dopant to improve the energy density of LiMn2O4 and enhance the average lithium intercalation voltage, the thermodynamics of Ni incorporation and its effect on the electrochemical properties of this spinel are not fully understood. Firstly, structural, electronic and mechanical properties of spinel LiMn2O4 and LiNixMn2-xO4 have been calculated out using density functional theory employing the pseudo-potential plane-wave approach within the generalised gradient approximation, together with Virtual Cluster Approximation. The structural properties included equilibrium lattice parameters; electronic properties cover both total and partial density of states and mechanical properties investigated elastic properties of all systems. Secondly, the pressure variation of several properties was investigated, from 0 GPa to 50 GPa. Nickel concentration was changed and the systems LiNi0.25Mn1.75O4, LiNi0.5Mn1.5O4 LiNi0.75Mn1.25O4 and LiNi0.875Mn1.125O4 were studied. Calculated lattice parameters for LiMn2O4 and LiNi0.5Mn1.5O4 systems are consistent with the available experimental and literature results. The average Mn(Ni)-O bond length for all systems was found to be 1.9 Å. The bond lengths decreased with an increase in nickel content, except for LiNi0.75Mn1.25O4, which gave the same results as LiNi0.25Mn1.75O4. Generally, analysis of electronic properties predicted the nature of bonding for both pure and doped systems with partial density of states showing the contribution of each metal in our systems. All systems are shown to be metallic as it has been previously observed for pure spinel LiMn2O4, and mechanical properties, as deduced from elastic properties, depicted their stabilities. Furthermore, the cluster expansion formalism was used to investigate the nickel doped LiMn2O4 phase stabilities. The method determines stable multi-component crystal structures and ranks metastable structures by the enthalpy of formation while iv maintaining the predictive power and accuracy of first-principles density functional methods. The ground-state phase diagram with occupancy of Mn 0.81 and Ni 0.31 generated various structures with different concentrations and symmetries. The findings predict that all nickel doped LMO structures on the ground state line are most likely stable. Relevant structures (Li4Ni8O16, Li12MnNi17O48, Li4Mn6Ni2O16, Li4Mn7NiO16 and Li4Mn8O16) were selected on the basis of how well they weighed the cross-validation (CV) score of 1.1 meV, which is a statistical way of describing how good the cluster expansion is at predicting the energy of each stable structure. Although the structures have different symmetries and space groups they were further investigated by calculating the mechanical and vibrational properties, where the elastic constants and phonon vibrations indicated that the structures are stable in accordance with stability conditions of mechanical properties and phonon dispersions. Lastly, a computer program that identifies different site occupancy configurations for any structure with arbitrary supercell size, space group or composition was employed to investigate voltage profiles for LiNixMn2-xO4. The density functional theory calculations, with a Hubbard Hamiltonian (DFT+U), was used to study the thermodynamics of mixing for Li(Mn1-xNix)2O4 solid solution. The results suggested that LiMn1.5Ni0.5O4 is the most stable composition from room temperature up to at least 1000K, which is in excellent agreement with experiments. It was also found that the configurational entropy is much lower than the maximum entropy at 1000K, indicating that higher temperatures are required to reach a fully disordered solid solution. The maximum average lithium intercalation voltage of 4.8 eV was calculated for the LiMn1.5Ni0.5O4 composition which correlates very well with the experimental value. The temperature has a negligible effect on the Li intercalation voltage of the most stable composition. The approach presented here shows that moderate Ni doping of the LiMn2O4 leads to a substantial change in the average voltage of lithium intercalation, suggesting an attractive route for tuning the cathode properties of this spinel. / National Research Foundation (NRF)

I processen att optimera och elektrifiera fordon som använder sig utav batterier har litiumjon battericeller introducerats till fordonen. Det vanligaste sättet är att tillverkaren installerar en stor battericell (> 10 Ah) i fordonen. En stor cell har många fördelar mot en liten cell, som att den är lättare att hantera, den utrustning som krävs för att övervaka cellen blir mindre och det krävs inga kopplingar mellan flertal celler. Det finns däremot en mängd fördelar med att ha mindre celler (< 5 Ah). De mindre cellerna skulle kunna bidra till en lägre kostnad, en jämnare värmefördelning över systemet och framförallt lättare att mekaniskt installera fordonet. Det vanligaste är att företag använder sig utav de större cellerna, det finns däremot fåtal exempel i privata fordonssektorn där tillverkare använder sig utav de mindre cellerna. Att använda sig utav de mindre cellerna kräver ett annat tänk när det gäller kylning, paketering i fordonen samt bevakningen av cellernas hårdvara och mjukvara blir annorlunda. Detta projekt har fokuserat på de elektriska och termiska aspekterna för implementering av parallellt kopplade små litiumjonceller i tunga fordon, som bussar och lastbilar. I projektet utfördes prestandaprov där temperatur, spänning och ström monitorerades över cellerna. Syftet var att öka kunskapen inom området för dessa små celler för att se om dessa har en potentiell plats på den kommersiella marknaden i framtiden. Målet med detta projekt är att mäta den spridning av ström som sker mellan de parallellt kopplade cellerna under variering av temperatur mellan cellerna. Från de utförda experimenten syns det tydligt att det sker en spridning av strömmen mellan cellerna. Den temperaturskillnaden som testas under experimentet påverkar inte strömmens spridning tillräckligt för att det ska visa någon differens i strömspridningen mellan cellerna. Detta ledde till att slutsatsen för projektet blir att det sker en strömspridning mellan parallellt kopplade celler, men temperaturdifferensen på tio grader celsius är inte tillräcklig för att påverka cellerna så pass att spridningen blir större. Under projektets gång möttes vi av många utmaningar och svårigheter. Detta har gjorde att den tid som kunde spenderas på provfasen blev väldigt kort. Det ufördes därför en minimal mängd av prov, vilket betyder att den data som samlades in under projektet inte var lika omfattande som det från början önskats. / In the process of optimizing and electrifying vehicles using batteries, lithium-ion battery cells have been introduced to the vehicles. The most common way is that the manufacturer installs a large battery cell (> 10 Ah) in the vehicles. A large cell has many advantages to a small cell. For example it is easier to handle, the equipment required to monitor the cell becomes smaller and no connections between multiple cells are required. On the other hand, there are many advantages of having smaller cells (<5 Ah). The smaller cells could contribute to a lower cost, a more even heat distribution across the system and, above all, easier to mechanically install in the vehicle. The most common choice for companies is to use the larger cells, but there are few examples in the private vehicle sector where manufacturers use the smaller cells. Using the smaller cells requires a different idea when it comes to cooling the cells, packing in the vehicles, and monitoring the hardware and software of the cells are different. This project focused on the electrical and thermal aspects of implementing parallel-connected small lithium-ion cells in heavy vehicles, such as buses and lorries. In this project performance tests were performed where temperature, voltage and current are monitored across the cells. The aim was to increase knowledge in the area of these small cells, to see if they have a potential place in the commercial market in the future. The goal of this project was to measure the spread of current that occurs between the parallel-connected cells during the varying temperature between the cells. From the experiments carried out, it was clear that there’s a spread of the current between the cells. The temperature difference tested during the experiment does not affect the spread of the current enough to show any difference in the current spread between the cells. Which leads to the conclusion of the project that there are a current spread between parallelconnected cells. However, the temperature difference of ten degrees Celsius is not sufficient to affect the cells enough that the spread becomes larger. The project faced a lot of challenges and difficulties. This has meant that the time spent on the experimental phase became very short. Therefore, a minimal amount of experiments was completed, which in turn means that the data collected for the project is not as extensive as it was meant to be initially.

The properties of C/SiCN nanocomposites synthesized by thermal decomposition of polymer precursors were studied in this work. The novel polymer-to-ceramic process enables us to tailor the ceramic structure in atomic level by designing the starting chemicals and pyrolysis procedures. It is of both fundamental and practical significance to investigate the properties and structures relationship of the nanocomposites. In this work, we explored their application potential in using as anode of lithium-ion secondary batteries. The structure and structural evolution of C/SiCN nanocomposite were investigated by using XRD, FTIR, SEM, TEM, Solid state NMR and Raman spectroscopy. The results revealed the nanocomposites consisted of amorphous SiCxNx-4 matrix and carbon nanoclusters distributed within it. The size of the carbon was measured by Raman spectroscopy, varied with starting chemicals and pyrolysis temperature. The electronic properties of the C/SiCN nanocomposite were studied by measuring the IV curves and a.c. impedance. The d.c. conductivity increased with carbon content and pyrolysis temperatures. The impedance spectra and fitted equivalent circuit results confirmed the existence of two phases in the nanocomposite. The possibility of using C/SiCN as anode in lithium-ion secondary batteries was investigated by electrochemical measurements, namely cyclic voltammetry, galvanostatic cyclic test and electrochemical impedance spectroscopy. The galvanostatic measurements showed that the nanocomposite with 26% of carbon nanoclusters exhibited a specific capacity of 480 mAh/g, iv which is 30% higher than that of commercial graphite anode. The high capacity of the nanocomposites is attributed to the formation of a novel structure around C/SiCN interface. The excellent electrochemical properties, together with the simple, low-cost processing, make the nanocomposites very promising for Li-ion battery applications

The increased demand for electric vehicles in recent years has driven the development of Li ion battery technology, yielding interesting trends in cathode materials. The layered cathode material Li(Ni1/3Mn1/3Co1/3)O2 gives 30% more reversible lithium extraction than the earlier LiCoO2 and the “overlithiated” material Li(Li0.2Mn0.54Ni0.13Co0.13)O2 gives a semi-reversible capacity 25% higher than Li(Ni1/3Mn1/3Co1/3)O2.1,2 6,7Li MAS NMR and 7Li MATPASS NMR were used to investigate the relation between the lithium ion and metal positions within these materials. It was found that Li(Ni1/3Mn1/3Co1/3)O2 showed a preference for Li ions to associate with Co at high voltages, that Mn4+ and Ni2+ showed some association and that the metals were not highly ordered. Li(Li0.2Mn0.5Ni0.13Co0.13)O2 showed a decrease in transition metal layer lithium upon cycling, in agreement with previous models, an ordering of the metal ions with the reinsertion of the lithium ions and a significant change in structure on deep discharge.3 These results will hopefully lead to more accurate modelling of the materials, understanding of reversibility and to increased reversible capacities in future cathode materials. Additionally work was done to enable high rate in-situ NMR spectra in which spectra are obtained from a cell while cycling in the bore of an NMR spectrometer. A Teflon Swagelok-style cell was designed and the effectiveness of solenoid and saddle coils were tested. It was found that for a 6 mm diameter cathode with a Li metal anode, at least half of the signal intensity could be obtained with a saddle coil whereas the signal was not detected when using a solenoid coil. / Thesis / Master of Science (MSc)

The field of energy storage has improved drastically within the last two decades. Batteries of various chemistries have been relied on to provide energy for numerous portable electronic devices. Lithium-ion cells, when compared to other chemistries have been known to provide outstanding energy-to-weight ratios and exhibit low self-discharge when not in use [1]. The aforementioned benefits in conjunction with decreasing costs have made lithium-ion cells an exceptional choice for use in electrical vehicles (EVs). Battery Management Systems (BMS) in EVs are responsible for providing estimates for values that are indicative of the battery pack’s present operating condition. The current operating condition could be described by State of Charge, power fade, capacity fade and various other parameters [2]. Importantly, it is essential for the estimation technique to adjust to fluctuating cell characteristics as the cell ages, in pursuance of having available accurate estimates for the life time of the pack. In order for the estimation technique to properly estimate the desired quantities, a mathematical model capable of capturing cell dynamics is desired. There are various proposed methods recommended for mathematically modeling a cell, namely equivalent Circuit modeling, electro-chemical modeling and impedance spectroscopy. Consequently, in order to ensure mathematical models are accurate and further to have the ability to compare the proposed models, it is essential to have available data gathered from a given cell at specific operating conditions. This Master’s thesis outlines the development of a lithium-ion cell tester that is capable of controlling, monitoring and recording parameters such as current, voltage and temperature. The Dual capability of obtaining data from standardized cell tests as well as high frequency cell tests is fascinating and intriguing. As this capability holds the possibility of reducing cost otherwise spent on man hours and equipment which are both paramount in any industrially automated process. / Thesis / Master of Applied Science (MASc)

Since the transportation sector wants to move away from fossil fuels and become more dependen ton clean and green energy, the use of rechargeable batteries is a good solution. Lithium-based batteries work very well as rechargeable batteries in electric vehicles. To extend the cycle life of lithium-based batteries a reversed Li+-trapping protocol has been implemented to extract lostcapacity. Prior to the start of this project, reversed Li+-trapping has been used on a proof-of-concept level and no specific protocol has been utilised in conjunction with full cells. With a protocol that uses constant voltage discharge steps on every fifth cycle, trapped Li+ can diffuse out from the graphite (anode) and travel back to the NMC811 (cathode). This will result in a lower capacity loss than if it only had cycled with constant current, using a C-rate of 1C. If a cell is operated with a constant voltage discharge step on every cycle, high capacity can be maintained for many cycles. This could potentially be used as a formation cycle protocol. The capacity is higher in the long run if the constant voltage discharge step is applied on every fifth cycle rather than every cycle. With a constant voltage discharge cell voltage of 2.8 V good results are obtained. A constant voltage discharge cell voltage of 0 V will however destroy the cell due to Cucurrent collector dissolution.

The work presented in this thesis can be divided into two research areas. First, two sets of organic-inorganic hybrid nanocomposites containing phenolic resin/trisilanolphenyl-POSS and phenolic resin/octa(aminophenyl)-T8-POSS nanocomposites were synthesized and the morphology and properties were investigated. Octa(aminophenyl)-T8-polyhedral silsesquioxane is an octafunctional-T8-POSS containing eight aniline-like amino groups, one on each corner silicon atom. It was synthesized in our laboratory by an improved two-step reaction sequence; nitration (HNO3) and reduction (HCOOH/Et3N). Varying amounts of POSS were codissolved with a resole phenolic resin in organic solvent. This was followed by solvent removal and thermal curing. Intermolecular interactions in these nanocomposites were probed by FT-IR. The micro-morphology and aggregation state of POSS were investigated using SEM, TEM, and WAXD studies. The thermal and mechanical properties and thermal stabilities of these composites were investigated by DMTA, DSC, and TGA. Second, two types of carbon-covered mono- and bimetallic (Sn, and Sn/Sb alloy) nanorods for use as anode materials in lithium ion batteries were synthesized by a thermal chemical vapor deposition method. Commercial antimony and tin oxide (Sb3O4/SnO2) nanopowders and added tin (IV) oxide (SnO2) nanoparticles (~19 nm) were used as the precursors for the growth of bimetallic Sn/Sb alloy and monometallic Sn nanorods, respectively. In addition, the shape of the products recovered were different when different hydrocarbon gas flow rates were used for growing intermetallic nanorods in carbon templates. Acetylene and methane were the gases tried. The morphologies and structures of the intermetallic nanorods in carbon templates were investigated using SEM and TEM and proved by X-EDS, XRD, and XPS studies.

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