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- Lundström, Robin
(författare)
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Exploring Reaction Pathways in Li-ion Batteries with Operando Gas Analysis
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The reliance on Li-ion batteries is increasing as we transition from fossil fuels to renewable energy sources. Despite their widespread use, a gap remains in understanding certain processes within these batteries, especially regarding the solid electrolyte interphase (SEI) and the impact of side reactions on Li-ion batteries. A custom-made Online Electrochemical Mass Spectrometry (OEMS) instrument was designed to explore these aspects. The OEMS instrument was validated through the study of gas-evolving reactions in the classic LiCoO2 | Graphite system. In-depth studies focusing on the reaction pathways of ethylene carbonate, the archetype Li-ion battery electrolyte solvent, identified the specific reaction pathways contributing to SEI formation. Moreover, ethylene carbonate’s interaction with residual contaminants like OH– from H2O reduction was explored. It was revealed that the integrity of the SEI can be compromised by minor amounts of contaminants, establishing a competitive dynamic at the negative electrode surface between ethylene carbonate and residual contaminants such as H2O and HF. Additionally, the roles of additives like vinylene carbonate and lithium bis(oxolato) borate in SEI formation were explored. Vinylene carbonate was shown to form a layer on the negative electrode, but also scavenge protons and H2O, revealing that it is a multi-functional additive. Lithium bis(oxolato) borate on the other hand formed an SEI layer before H2O reduction, blocking the residual contaminant and ethylene carbonate from reaching the electrode surface. By providing insights into the negative electrode’s interphase and SEI formation through a custom-made OEMS instrument, this research underscores the complexity of reaction pathways and the necessity of considering both major and minor, as well as, primary and secondary reactions for a holistic understanding of Li-ion batteries.
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| 2. |
- Ma, Le Anh, 1992-
(författare)
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Anti-Ageing Strategies : How to avoid failure in sodium-ion batteries
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In order to move away from fossil fuels, batteries are one of the most important technologies to store energy from renewable sources. The rapid demands of battery applications put pressure on supply chains of raw materials, such as lithium, nickel, copper, aluminium and cobalt. There is a concern about the availability of such elements in the future. Sodium-ion batteries based on naturally abundant elements have become an attractive alternative to lithium-ion batteries due to their potential to reduce the cost and to improve the sustainability of batteries. A low electrochemical cycling stability of these Na-ion batteries can hinder long-term implementation in large-scale applications. It is necessary to understand what can lead to ageing and electrochemical cycling failure in sodium-ion batteries and how such detrimental side-reactions can be prevented. Compared to lithium-ion batteries, the research on sodium-ion batteries is not as mature yet.This thesis work sheds light on the ageing mechanisms at the electrode/electrolyte interfaces and in the bulk of electrode materials with the help of a variety of spectroscopic and electrochemical methods. The electrochemical properties at the anode/electrolyte interface have been carefully investigated with different galvanostatic cycling protocols and x-ray photoelectron spectroscopy (XPS). The solid electrolyte interphase (SEI) in sodium-ion batteries is known to be inferior to its Li-analogue and hence, its long-term stability needs to be thoroughly investigated in order to improve it. Fundamental properties of the SEI in regards to formation, growth and dissolution are investigated on platinum and carbon black electrodes in different electrolyte systems. As well as the use of unconventional additives have proven to saturate the electrolyte and to mitigate SEI dissolution. This work shows one of the few studies highlighting SEI dissolution using electrochemical cycling tests coupled with pauses, in order to detect SEI ageing in batteries. Ageing mechanisms in manganese-based cathodes have also been studied due to the abundance of manganese and their electrochemical performance at high voltages with synchrotron-based XPS, x-ray absorption spectroscopy (XAS), resonant inelastic x-ray scattering (RIXS) and muon spin relaxation measurements coupled with electrochemical techniques. Surface-sensitive studies revealed how capacity losses stem from electrolyte degradation which results in a redox gradient between surface and bulk electrode. The work also shows how anionic redox contributions and incomplete phase transitions are reasons of additional capacity losses observed in manganese-based cathodes. Furthermore, it shows how a low Na-mobility is also an indicator for inferior long-term cycling properties leading capacity losses.
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| 3. |
- Mogensen, Ronnie
(författare)
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Realization of Sodium-ion Batteries : From Electrode to Electrolyte Materials
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Batteries are among the most important technologies required to enable the world to move beyond fossil fuels towards a more efficient and environmentally friendly society based on electricity from renewable sources. Unfortunately, the rapidly increasing number and size of batteries that the world needs in order to perform this paradigm shift is putting enormous strain on the supply of traditional raw materials for batteries, such as lithium and cobalt. Batteries built using only earth abundant elements could guarantee that the supply of energy storage will be available to everyone at reasonable prices. Sodium-ion batteries are among the most popular candidates to achieve battery systems that can provide performance close to or on par with lithium-ion batteries at a lower cost and environmental impact. Although the sodium-ion and lithium-ion batteries share many properties, there is a lot of research required before sodium-ion batteries can compete with the highly optimised lithium-ion batteries. This work explores the stability of the solid electrolyte interphase (SEI) formed on the anode in sodium-ion batteries through means of electrochemical measurements and x-ray photoelectron spectroscopy (XPS) analysis. The fundamental properties in regards to solubility and electrochemical stability of the surface layer on model anodes as well as on anode materials like hard carbon and tin-phosphide is discussed. The synthesis and electrochemical performance of Prussian white comprising of all earth abundant elements for use as a low-cost and high-performance cathode material is demonstrated. The work also includes several investigations of alternative solvents and salts for electrolytes that have been analysed in conjunction with sodium-ion cells based on hard carbon and Prussian white. The electrolytes studied possess a wide spectrum of different opportunities such as high ionic conductivity, non-flammability, fluorine-free composition and improved low and high-temperature performance.
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| 4. |
- Nilsson, Viktor, 1985-
(författare)
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Highly Concentrated Electrolytes for Rechargeable Lithium Batteries
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The electrolyte is a crucial part of any lithium battery, strongly affecting longevity and safety. It has to survive rather severe conditions, not the least at the electrode/electrolyte interfaces. Current commercial electrolytes are almost all based on 1 M LiPF6 in a mixture of organic solvents and while these balance the many requirements of the cells, they are volatile and degrade at temperatures above ca. 70°C. The salt could potentially be replaced with e.g. LiTFSI, but dissolution of the Al current collector would be an issue. Replacing the graphite electrode by Li metal, for large gains in energy density, challenges the electrolyte further by exposing it to freshly deposited Li, leading to poor coulombic efficiency and consumption of both Li and electrolyte. Highly concentrated electrolytes (HCEs) have emerged as a possible remedy to all of the above, by a changed solvation structure where all solvent molecules are coordinated to cations – leading to a lowered volatility, a reduced Al dissolution, and higher electrochemical stability, at the expense of higher viscosity and lower ionic conductivity.In this thesis both the fundamentals and various approaches to application of HCEs to lithium batteries are studied. First, LiTFSI–acetonitrile electrolytes of different salt concentrations were studied with respect to electrochemical stability, including chemical analysis of the passivating solid electrolyte interphases (SEIs) on the graphite electrodes. However, some problems with solvent reduction remained, why second, LiTFSI–ethylene carbonate (EC) HCEs were employed vs. Li metal electrodes. Safety was improved by avoiding volatile solvents and compatibility with polymer separators was proven, making the HCE practically useful. Third, the transport properties of HCEs were studied with respect to salt solvation, viscosity and conductivity, and related to the rate performance of battery cells. Finally, LiTFSI–EC based electrolytes were tested vs. high voltage NMC622 electrodes.The overall impressive electrochemical stability improvements shown by HCEs do not generally overcome the inherent properties of the constituent parts, and parasitic reactions ultimately leads to cell failure. Furthermore, improvements in ionic transport can not be expected in most HCEs; on the contrary, the reduced conductivity leads to a lower rate capability. Based on this knowledge, turning to a concept of electrolyte compositions where the inherent drawbacks of HCEs are circumvented leads to surprisingly good electrolytes even for Li metal battery cells, and with additives, Al dissolution can be prevented also when using NMC622 electrodes.
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| 5. |
- Tesfamhret, Yonas
(författare)
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Transition metal dissolution from Li-ion battery cathodes
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Lithium-ion batteries (LIBs) have become reliable electrochemical energy storage systems due to their relative high energy and power density, in comparison to alternative battery chemistries. The energy density of current LIBs is limited by the average operating voltage and capacity of oxide-based cathode materials containing a variety of transition metals (TM). Furthermore, the low anodic stability of "conventional" carbonate-based electrolytes limits further extension of the LIBs voltage window. Here, ageing mechanisms of cathodes are investigated, with a main focus on TM dissolution and on strategies to tailor the cathode surface and the electrolyte composition to mitigate TM dissolution.Atomic layer deposition (ALD) coatings of the cathode surface with electrically insulating Al2O3 and TiO2 coatings is employed and investigated as a method to stabilize the cathode/electrolyte interface and minimize TM dissolution. The thesis illustrates both the advantages and limitations of amorphous oxide coating materials during electrochemical cycling. The protective oxide layer restricts auto-catalytic salt degradation and the consequent propagation of acidic species in the electrolyte. However, a suboptimal coating contributes to a nonhomogeneous cathode surface ageing during electrochemical cycling. Furthermore, the widely accepted concept of charge disproportionation as the fundamental cause of TM dissolution is demonstrated to be a minor factor. Rather, a chemical dissolution mechanism based on acid-base/electrolyte-cathode interaction underlies substantial TM dissolution.The thesis demonstrates LiPF6, and by implication HF, as the principal source of TM dissolution. In addition, the oxidative degradation of ethylene carbonate (EC) solvent contributes indirectly to generation of HF. Thus, an increase in electrolyte oxidative degradation products accelerates TM dissolution. Substituting EC and LiPF6 with a more anodically stable solvent (e.g., tetra-methylene sulfone) and a non-fluorinated salt (e.g., LiBOB or LiClO4) or addition of TM scavenging additives like lithium difluorophosphate (LiPO2F2) are here investigated as strategies to either i) mitigate TM dissolution, ii) supress TM migration and deposition on the anode surface, or iii) supress formation of acidic electrolyte degradation products and thereby TM dissolution. The thesis also highlights the necessity of taking precautions when attempting to replace the components, as reducing TM dissolution may come at the expense of electrochemical cycling performance.
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