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- Källquist, Ida
(författare)
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Interfaces in Li-ion batteries seen through photoelectron spectroscopy
- 2019
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- To accommodate the need for greener energy solutions renewable energy sources aswell as reliable energy storage is a prerequisite. For the latter, high energy densitybatteries with long-term cycling stability are necessary. The cycling properties of abattery is to a large extent dependent on the functionality of the battery interfaces. Assuch, there is a need to understand the reactions occurring between the electrode andelectrolyte, and to limit those that are detrimental to the battery performance. Thetopic of this thesis is these interfaces in Li-ion batteries seen through photoelectronspectroscopy (PES).PES is due to its surface and chemical sensitivity one of the most suitable techniquesto study battery interfaces. In this thesis, PES is used to follow the oxidationstate and chemical environment of different atoms to understand the reactions occurringin the battery. This work uses a combination of soft and hard X-ray photoelectronspectroscopy as well X-ray absorption spectroscopy (XAS) to investigate the degradationmechanisms in high energy density cathode materials. The materials investigatedare in the class of Li-rich disordered rock-salts (DRS) and provide very highinitial capacities, but unfortunately lacks in cycling stability. In this thesis it is shownthat the reason for this is an unstable surface, possibly related to the occurrence ofanionic redox in the material, leading to breakdown of both electrolyte and electrodematerial. In addition, it is shown that the interface stability can be improved by choosingtransition metals that promotes the DRS structure and thus increases the chemicalstability of the material and long term cycling of the battery.Even though ex situ measurements provide many insights into the properties ofbattery interphases, there is still a need for operando measurement to completely answerthe puzzling question of their full functionality. In this thesis first steps towardsoperando measurements are taken by identifying the measurements conditions necessaryto probe a battery electrolyte with ambient pressure photoelectron spectroscopy(APPES) and a thorough characterization of a typical battery electrolyte is performed.The results show that the liquid can be stabilized by using the solvent as ambient gas,and also that care should be taken to avoid radiation damage when synchrotron lightis used. For the electrolyte characterization it is shown that a salt enrichment of particularlyLi+ and ionic fluoride is found at the droplet surface. These results are crucialto be able to single out contributions from the interphase in future operando measurements.When the method of operando APPES has matured and can be performed routinely,this could possibly be the key needed to understand how the interfaces in batteriescan be controlled to unlock the potential of stable high capacity materials infuture batteries.
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| 2. |
- Nilsson, Viktor, 1985-
(författare)
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Highly Concentrated Electrolytes for Lithium Batteries : From fundamentals to cell tests
- 2018
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Licentiatavhandling (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 based on 1 M LiPF 6 in a mixture of organic solvents balance the requirements on conductivity and electrochemical stability, but they are volatile and degrade when operated at temperatures above ca. 70°C. The salt could potentially be replaced with e.g. LiTFSI, but corrosion of the aluminium current collector is an issue. Replacing the graphite negative 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 (CE) and consumption of both Li and electrolyte. Highly concentrated electrolytes (up to > 4 M) have emerged as a possible remedy, by a changed solvation structure such that all solvent molecules are coordinated to cations – leading to a lowered volatility and melting point, an increased charge carrier density and electrochemical stability, but a higher viscosity and a lower ionic conductivity.Here two approaches to highly concentrated electrolytes are evaluated. First, LiTFSI and acetonitrile electrolytes with respect to increased electrochemical stability and in particular the passivating solid electrolyte interphase (SEI) on the anode is studied using electrochemical techniques and X-ray photoelectron spectroscopy. Second, lowering the liquidus temperature by high salt concentration is utilized to create an electrolyte solely of LiTFSI and ethylene carbonate, tested for application in Li metal batteries by characterizing the morphology of plated Li using scanning electron microscopy and the CE by galvanostatic polarization. While the first approach shows dramatic improvements, the inherent weaknesses cannot be completely avoided, the second approach provides some promising cycling results for Li metal based cells. This points towards further investigations of the SEI, and possibly long-term safe cycling of Li metal anodes.
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