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- Ensling, David, et al.
(author)
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A comparative XPS surface study of Li2FeSiO4/C cycled with LiTFSI- and LiPF6-based electrolytes
- 2009
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In: Journal of Materials Chemistry. - : Royal Society of Chemistry (RSC). - 0959-9428 .- 1364-5501. ; 19, s. 82-88
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Journal article (peer-reviewed)abstract
- X-Ray photoelectron spectroscopy (XPS) has been used to characterise the surfaces of carbon-coated Li2FeSiO4 cathodes extracted from Li-ion batteries in both a charged and discharged state. 1 M lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and lithium hexafluorophosphate (LiPF6) based electrolytes were used with ethylene carbonate (EC) and diethyl carbonate (DEC) as organic solvents. The LiTFSI-based electrolyte exhibited high salt stability and no significant formation of LiF. However, solvent reaction products from EC were found together with lithium carbonate. A LiPF6-based electrolyte, on the other hand, showed inferior salt stability with LixPFy, LixPOyFz and LiF species formed on the surface. Solvent reaction products together with lithium carbonate were also found. There are also indications that Li2FeSiO4 is degraded by the HF formed in the electrolyte by the hydrolysis of LiPF6. A better understanding of the surface chemistry of carbon-coated Li2FeSiO4 after the first cycles in a Li-ion battery has thus been achieved, thereby facilitating the optimisation of Li-ion batteries based on this potentially cheap and electrochemically most promising cathode material giving excellent capacity retention: <3% drop over 120 cycles.
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| 6. |
- Nytén, Anton, et al.
(author)
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Surface characterization and stability phenomena in Li2FeSiO4 studied by PES/XPS
- 2006
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In: Journal of Materials Chemistry. - : Royal Society of Chemistry (RSC). - 0959-9428 .- 1364-5501. ; 16:34, s. 3483-3488
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Journal article (peer-reviewed)abstract
- Photoelectron spectroscopy (PES) has been used to characterise the surface of Li2FeSiO4 cathodes extracted from lithium-ion batteries. Pristine, uncycled, air-exposed electrodes were first analysed and found to carry significantly greater amounts of Li2CO3 on their surfaces than electrodes stored under inert atmosphere. The surface film formed on electrochemical cycling of Li2FeSiO4 electrodes at 60 degrees C using a LiN(SO2CF3)(2) salt based electrolyte revealed high salt stability and only small amounts of solvent reaction products. These were mainly of Li-carboxylate type; neither carbonates nor LiF were found. The excellent capacity retention (<3% over 120 cycles) and minimal irreversible capacity during the first cycle are probably a direct result of this very thin surface film. Li2FeSiO4 must therefore be seen as a most promising ( and potentially cheap) positive electrode material for future large-scale Li-ion battery applications.
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- Stjerndahl, Mårten, 1978-
(author)
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Stability Phenomena in Novel Electrode Materials for Lithium-ion Batteries
- 2007
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Doctoral thesis (other academic/artistic)abstract
- Li-ion batteries are not only a technology for the future, they are indeed already the technology of choice for today’s mobile phones, laptops and cordless power tools. Their ability to provide high energy densities inexpensively and in a way which conforms to modern environmental standards is constantly opening up new markets for these batteries. To be able to maintain this trend, it is imperative that all issues which relate safety to performance be studied in the greatest detail. The surface chemistry of the electrode-electrolyte interfaces is intrinsically crucial to Li-ion battery performance and safety. Unfortunately, the reactions occurring at these interfaces are still poorly understood. The aim of this thesis is therefore to increase our understanding of the surface chemistries and stability phenomena at the electrode-electrolyte interfaces for three novel Li-ion battery electrode materials. Photoelectron spectroscopy has been used to study the surface chemistry of the anode material AlSb and the cathode materials LiFePO4 and Li2FeSiO4. The cathode materials were both carbon-coated to improve inter-particle contact. The surface chemistry of these electrodes has been investigated in relation to their electrochemical performance and X-ray diffraction obtained structural results. Surface film formation and degradation reactions are also discussed. For AlSb, it has been shown that most of the surface layer deposition occurs between 0.50 and 0.01 V vs. Li°/Li+ and that cycling performance improves when the lower cut-off potential of 0.50 V is used instead of 0.01 V. For both LiFePO4 and Li2FeSiO4, the surface layer has been found to be very thin and does not provide complete surface coverage. Li2CO3 was also found on the surface of Li2FeSiO4 on exposure to air; this was found to disappear from the surface in a PC-based electrolyte. These results combine to give the promise of good long-term cycling with increased performance and safety for all three electrode materials studied.
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