| 1. |
- Gudla, Harish
(författare)
-
Using Molecular Dynamics Simulations to Explore Critical Property Relationships in Polymer Electrolytes : Polarity, Coordination, Ionic transport, Ion-pairing, and Ion-ion Correlations
- 2022
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- While ion transport in solid polymer electrolytes (SPEs) has been explored for decades, there still remains controversies about its fundamental properties, often correlated with gaps between experimental and computational studies. Using molecular dynamics simulations to understand the complex transport mechanisms and also to fill these gaps is the main goal of this thesis. This is achieved by critically examining the relationships between different properties in SPE systems: polarity, coordination, ion-pairing, and ion-ion correlations, which highly influence the ionic transport mechanism. Firstly, the relation between polarity, ion-pairing, and ion-ion correlations was explored. The solvent polarity (εp) of poly(ethylene oxide) (PEO) doped with LiTFSI system is modulated using a charge scaling method. When separating the effects of solvent polarity and glass transition temperature, a maximum in the Li-ion diffusion coefficient with respect to εp is observed. This is attributed to the transitions in the transport mechanisms and an optimal solvating ability of Li-ion at intermediate values of εp. The solvent polarity also plays a critical role in the formation of charge-neutral ion pairs, which is commonly considered detrimental for ionic conductivity. The relation between cation−anion distinct conductivity and the lifetime of ion pairs was thereby examined, where it is found that short-lived ion pairs actually contribute positively to the ionic conductivity. Moreover, the origins of the recently observed negative transference numbers were scrutinized. A strong dependence of the reference frame in the estimation of the transference numbers is found, which explains observed differences between experiments and computations. Secondly, the role of coordination chemistry and its influence on ion transport mechanisms and conduction properties in SPEs was studied. The change in the cation coordination with both polymers and anions was used to study the dominant transport mechanisms at different molecular weights and salt concentrations for PEO and a polyester-based SPE, which shows that essentially very little true hopping occurs in these materials. In this context, the coordination and ionic transport properties of three resemblant carbonyl-coordinating polymers are also investigated: polyketones, polyesters, and polycarbonates. The extra main-chain oxygens for the latter polymers are shown to decrease the electrostatic energy between Li-ion and the carbonyl group, and the cationic transference numbers are thus found to be increasing as the coordination strength decrease.
|
|
| 2. |
- Oltean, Alina, 1987-
(författare)
-
Building Sustainable Batteries : Organic electrodes based on Li- and Na-benzenediacrylate
- 2018
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- As possible alternatives to the conventional inorganic Li- or Na-ion battery electrode materials, organic compounds have recently drawn considerable attention. However, major challenges such as poor electronic conductivity, solubility in battery electrolyte or fast capacity decay of the resulting electrochemical cells are some of the reasons that hold these compounds back from becoming commercial solutions in the energy system.The goal of this thesis work was to investigate the background to these phenomena and find strategies for improvements. Two different compounds were studied: dilithium and disodium benzenediacrylate, in their respective cells. First, improving the performance of the dilithium compound was performed by applying different electrode fabrication strategies. A freeze-drying technique was combined with carbon coating in the liquid state, which rendered an improved electrode morphology. Moreover, when using the compound in pouch cell format instead of Swagelok® cells, a different technique was applied: calendaring. Successful results were obtained both in half-cells and in full-cells when the compound was cycled versus LiFePO4-based cathodes. Second, the sodium analogue was investigated, and while the synthesis of this compound is straightforward, the electrochemical performance in Na-ion battery cells displays an unexpected degree of complexity. The compound displays a considerably faster capacity decrease in comparison to the Li compound, and generally a poor chemical stability in the applied system. When cycled at higher currents (C-rates of C/4 or C/10, in comparison to C/40), the compound presents an capacity increase while the Li decreases, likely due to a chemical process more dependent on time than on the number of cycles for the Na compound.The fast capacity decay in the first cycles of these types of compounds is often considered to be related to the Solid Electrolyte Interphase (SEI) layer formation. Its study was also performed and it was concluded that the Na compound has a thicker SEI layer in comparison to the Li counterpart, and mostly consisted of inorganic species such as the electrolyte salt and its decomposition products. Finally, a concept for a sustainable manufacturing and recycling process of a hybrid full cell is also performed with positive results.Although the organic compounds cannot yet outperform the inorganic compounds used commercially in Li-ion batteries, important steps towards their employment in the energy system have been taken in this thesis work.
|
|
| 3. |
- Björklund, Erik
(författare)
-
Avoiding ageing : Surface degradation of commercial electrode materials in lithium-ion batteries
- 2019
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The battery market today expands rapidly, not least for electric vehicles. But to compete against the combustion engine, the cost of batteries must be reduced. After years of usage, the batteries degrade and need to be exchanged, increasing the cost over the vehicle lifecycle. This can be mitigated by tailoring the usage conditions and the battery materials. Understanding and avoiding ageing can be key to a more sustainable transport system. This thesis contains studies on degradation processes in Li-ion batteries utilizing the LiNixMnyCozO2 (NMC) cathode material, and suggests strategies for the improvement of battery life time.When cycling different negative electrodes – including graphite, lithium foil and lithium titanium oxide (LTO) – against NMC electrodes, only minor capacity fading was observed in the NMC-LTO and NMC-graphite cells, in contrast to the NMC-Li-metal cells. The capacity fading for Li-metal cells was determined to be caused by degradation products formed at the lithium foil which thereafter diffused to the NMC electrode, leading to a higher resistance. Commercial NMC/LiMn2O4-graphite cells were also investigated after cycling in limited state of charge (SOC)-intervals. The cycle life was far longer in the low-SOC cell than in the high-SOC cell. Photoelectron spectroscopy revealed increased manganese dissolution in the high-SOC cell, likely causing a less stable solid electrolyte interphase layer on the negative electrode. This, in turn, limits the capacity. How temperature influence ageing in NMC-LTO was analysed in cells cycled at -10 °C, 30 °C and 55 °C. It was found that the initial side reactions at the LTO electrode limited the cell capacity, but that these also stabilized the NMC electrode. At 55 °C, excessive side reactions at LTO caused capacity fading due to loss of active lithium. At -10 °C, high cell resistance limited the capacity. Switching to a PC based electrolyte allowed stable low temperature cycling, although it was found that PC degraded and formed thick electrode surface layers. Also sulfolane-based electrolytes were investigated, showing thinner surface layers than the EC containing reference electrolyte at high potentials, thus indicating a more stable electrolyte system.
|
|
| 4. |
- Carvalho, Rodrigo P.
(författare)
-
Organic Electrode Battery Materials : A Journey from Quantum Mechanics to Artificial Intelligence
- 2022
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Batteries have become an irreplaceable technology in human life as society becomes progressively more dependent on electricity. The demand for novel battery technologies has increased fast, especially with the popularisation of different portable devices. However, the current battery industry relies heavily on non-renewable resources that are also prone to provoke environmental harm. Among the possible candidates for the next generation of batteries, organic electroactive materials (OEMs) have become attractive due to a series of advantages: vastly accessible from renewable raw materials; highly versatile due to the possible functionalisation mechanisms; possibly lower production costs; reduced environmental impacts; etc. Nevertheless, some drawbacks need to be overcome before OEMs become competitive. Issues with energy density, rate capability and cycling stability hinder their final technological application. This thesis thereby discusses fundamental aspects of OEMs and proposes novel techniques to accelerate the materials discovery process.The first part of this thesis presents a pathway to systematically investigate organic materials by combining quantum mechanics calculations and crystal structure predictions. An evolutionary algorithm predicts the crystal structure of several OEMs, enabling an initial assessment of the electronic structure and the thermodynamics of the ionic insertion mechanism in these compounds. Furthermore, this first part also suggests an approach to tailor OEMs, identifying their charge storage limits and the possible occurrence of metastable phases during the ion insertion process. However, the presented strategy, while accurate, is seriously limited by its high computational demands, which are unrealistic for high-throughput screening of novel materials.Since organic materials represent a possibly limitless universe of compounds, alternative techniques are needed. Thus, the second part of this thesis combines quantum mechanics and artificial intelligence (AI), rendering a powerful platform to aid this task. An “AI-\textit{kernel}” was employed to analyse millions of organic compounds, discovering novel possible organic battery materials. Moreover, the AI accurately identified common functional groups associated with higher-voltage electrodes and suggested features that may aid future materials design. Furthermore, the kernel can also identify materials suitable for Na- and K-ion batteries and anticipate their redox stability.In conclusion, this thesis has focused on investigating fundamental properties of organic electroactive materials, particularly the ionic insertion process in batteries. Furthermore, AI-driven methodologies have also been proposed, accurately evaluating OEMs and enabling fast access to the gigantic organic realm when searching for novel battery electrode materials.
|
|
| 5. |
- Hedman, Jonas, 1992-
(författare)
-
Fiber Optic Sensors for Monitoring of Lithium- and Sodium-ion Batteries
- 2022
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Rechargeable batteries, particularly lithium-ion batteries, have rapidly evolved since their introduction and now dominate the market, owing primarily to their high energy and power densities. With growing demand for high-performance batteries in portable electronics and electric vehicles, the need for safe, efficient, and reliable batteries is crucial. Conventional battery management systems, which generally rely on parameters such as current, voltage, and temperature, provide limited information on the chemical and physical processes taking place in the battery during operation. The understanding of degradation processes and how they evolve with time is also limited due to the complex nature of batteries. In order to enhance the battery lifetime, safety, and reliability of current batteries as well as for emerging battery technologies, more detailed information from the cells is required. Developing sensors that can be used to probe the batteries could allow for optimized performance and a more accurate determination of cell state. In this regard, fiber optic sensors are promising candidates.This work explores the use of fiber optical evanescent wave (FOEW) sensors for monitoring chemical and electrochemical reactions in lithium- and sodium-ion batteries under working conditions. The sensor response and battery performance were compared with the sensor either fully embedded in a lithium iron phosphate cathode or positioned at the electrode surface. The optical response was further linked to the oxidation and reduction of the active material during cycling by means of galvanostatic and voltammetric experiments. The influence of cycling rate, sensor position, and electrolyte salt concentration was also discussed. The work also shows the ability of the FOEW sensors to detect lithium and sodium plating, both as a result of insufficient storage capacity and high cycling rates. This is an important finding as plating poses a serious risk for short circuit in batteries. A correlation with the sensor response and lithium staging in graphite anodes could also be seen.These findings highlight the value of optical sensors for monitoring batteries under working conditions. The concept of fiber optic sensing in batteries is still in its early stages, but the research field is gaining more interest. This work has aimed to advance the understanding of FOEW sensors in particular, and the results could help to provide directions for the research community for the realization of fiber optic sensing in commercial batteries.
|
|
| 6. |
- Jeschull, Fabian, 1989-
(författare)
-
Polymers at the Electrode-Electrolyte Interface : Negative Electrode Binders for Lithium-Ion Batteries
- 2017
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
|
|
| 7. |
- Karo, Jaanus, 1977-
(författare)
-
The Rôle of Side-Chains in Polymer Electrolytes for Batteries and Fuel Cells
- 2009
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The subject of this thesis relates to the design of new polymer electrolytes for battery and fuel cell applications. Classical Molecular Dynamics (MD) modelling studies are reported of the nano-structure and the local structure and dynamics for two types of polymer electrolyte host: poly(ethylene oxide) (PEO) for lithium batteries and perfluorosulfonic acid (PFSA) for polymer-based fuel cells. Both polymers have been modified by side-chain substitution, and the effect of this on charge-carrier transport has been investigated. The PEO system contains a 89-343 EO-unit backbone with 3-15 EO-unit side-chains, separated by 5-50 EO backbone units, for LiPF6 salt concentrations corresponding to Li:EO ratios of 1:10 and 1:30; the PFSA systems correspond to commercial Nafion®, Hyflon® (Dow®) and Aciplex® fuel-cell membranes, where the major differences again lie in the side-chain lengths.The PEO mobility is clearly enhanced by the introduction of side-chains, but is decreased on insertion of Li salts; mobilities differ by a factor of 2-3. At the higher Li concentration, many short side-chains (3-5 EO-units) give the highest ion mobility, while the mobility was greatest for side-chain lengths of 7-9 EO units at the lower concentration. A picture emerges of optimal Li+-ion mobility correlating with an optimal number of Li+ ions in the vicinity of mobile polymer segments, yet not involved in significant cross-linkages within the polymer host.Mobility in the PFSA-systems is promoted by higher water content. The influence of different side-chain lengths on local structure was minor, with Hyflon® displaying a somewhat lower degree of phase separation than Nafion®. Furthermore, the velocities of the water molecules and hydronium ions increase steadily from the polymer backbone/water interface towards the centre of the proton-conducting water channels. Because of its shorter side-chain length, the number of hydronium ions in the water channels is ~50% higher in Hyflon® than in Nafion® beyond the sulphonate end-groups; their hydronium-ion velocities are also ~10% higher.MD simulation has thus been shown to be a valuable tool to achieve better understanding of how to promote charge-carrier transport in polymer electrolyte hosts. Side-chains are shown to play a fundamental rôle in promoting local dynamics and influencing the nano-structure of these materials.
|
|
| 8. |
- Sångeland, Christofer, 1991-
(författare)
-
Exploring the Frontiers of Polymer Electrolytes for Battery Applications : From Surface to Bulk
- 2021
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Lithium-ion batteries have dominated the market since their inception in 1991 due to their unparalleled energy and power densities, but are now faced with new challenges. Growing demand for battery materials for energy intense applications and large-scale interim energy storage have emphasized the need for safe and sustainable battery electrolytes. In this context, non-flammable solid polymer electrolytes (SPEs) are a promising alternative to address the shortcomings of conventional liquid electrolytes. Despite its significance, little research has thus far been devoted to understanding the electrochemical stability of SPEs under the harsh conditions exerted by next-generation electrode materials.In this thesis, the stability and ramifications of interfaces in polycarbonate- and polyester-based SPEs have been investigated. The polycarbonate exhibited severe degradation upon contact with lithium compared to its ester counterpart. Volatile species stemming from polycarbonate and salt decomposition were observed independent of irreversible current response, thus also highlighting the limitations of voltammetry techniques to determine the electrochemical stability. Two novel techniques were thus devised to evaluate electrochemical stability of SPEs under more realistic conditions. Characterization of the electrode−polyester interface revealed formation of highly resistive interfacial layers composed of polymer, salt and impurity derivatives. The emergence of a detrimental resistance emanating from the polymer−polymer interface was also observed, thus identifying a crucial hurdle for double-layer SPEs as a strategy to extend the stability window.The application of polycarbonate/polyester-based polymer electrolytes for sodium-ion batteries was also studied. Sodium is far more abundant than lithium, and thereby an excellent chemistry platform to develop new sustainable battery materials. The polycarbonate exhibited an exceptional ability to dissolve large quantities of sodium salt without compromising the mechanical stability. Spectroscopic and thermal measurements revealed the emergence of an alternative ionic transport mechanism at concentrations within the polymer-in-salt regime, which was decoupled from the segmental motion of the polymer chains. By incorporating flexible polyester moieties in polycarbonates, an SPE with better transport properties compared to its individual subunits, and polyether counterparts, was obtained. Optimal salt concentration in this copolymer was dependent on the degree of crystallinity, determined by the portion of polyester. Finally, the practical application of these polymer electrolytes was demonstrated in solid-state sodium-ion batteries.
|
|
