| dc.description.abstract | Li-ion battery performance is substantially dependent on the SEI properties on the graphite anode. This is easily understood if we consider that the performance parameters such as cycle life and temperature window always involve (direct/indirect) reference to the SEI functionality. Accordingly, the ultimate aim of this work was to explore the possibilities that can stand up to the demands raised by not only scientific circles, but also by the technological applications of the Li-ion batteries. We were especially interested in the Li+- ion solvation shell consisting of organic solvent(s), EC(DMC), and/or inorganic anion (PF6-), which may be taken as the first step of the SEI formation and perhaps intercalation. This may seem at first sight to contradict the well-established definitions of the SEI formation, however this is being verified by current research front that modifies and therefore revises the old models. This lead us to identifying two electrolyte additives that can alter and reorient the Li+ solvation shell. The additives identified were; i) anion receptor (THFIPB), and ii) NaPF6. To our surprise the amount of the work available in this space was limited to a few groups, mainly in the USA and Japan.
The rationale behind the use of anion receptor was to explore whether the anion of the electrolyte salt would manifest a tendency to modify the SEI chemistry by changing the Li+ solvation, i.e., ion-pairs. Perhaps the correct way of seeing it would be to look at the anion receptor as a solvent for the anion, which is known to be weakly-solvated by the electrolyte solvents, mainly organic carbonates. Dual-carbon-cells are certainly another instance of use of such anion receptors. Based on previous work, we confined our search for a suitable anion receptor to those with relatively high binding energies with negative ion and identified Tris (hexafluoroisopropyl) borate (THFIPB) for further study.
Similarly, we chose NaPF6 as the other additive to rework the Li+···EC(DMC) structure with the main goal to potentially enable reversible Li+ intercalation into graphite in a PC based electrolyte. Although we hypothesized that there would be competition between Li+ and Na+ for EC solvation, it was not clear whether this new circumstance in the electrolyte would favor reversible Li+ intercalation by reducing the solvation number of the Li+ ions by the cyclic carbonates.
Both additives were therefore essentially used to reorganize the Li+ ion solvation shell and eventually to tune the interface reactions, including SEI formation and desolvation of Li+ ion from its solvation shell at the electrolyte/SEI and the graphite/SEI interfaces. I think, however, it is necessary to emphasize the fundamental difference between them. Anion receptor binds the anion, and thus releases Li+ ion from the ion-pairs. NaPF6 on the other hand causes reduction in the solvation number of the Li+ ion and does not necessarily impact the ion-pairs.
The first paper strictly concentrates on the addition of the anion receptor in a standard commercial electrolyte consisting of 1M LiPF6 in EC:DMC (1:1 wt.%). In this paper we evaluated the electrochemistry of the anion receptor (AR) containing electrolyte on commercial SLP30 graphite. SEI formation and morphology, salt and electrolyte reductions, and Li+ intercalation were monitored during galvanostatic cycles. Post-mortem SEI morphology was elaborated without exposing the graphite to air atmosphere in order to ensure the Li metal plating phenomenon be observed. Rate tests, electrochemical impedance spectroscopy and in-situ XRD were carried out to come to highlight the difference between graphite electrochemistry in commercial electrolyte and anion receptor added electrolyte. It was found that the anion receptor addition significantly improved the intercalation kinetics in the first charging of the electrode with ~25 mV more positive intercalation plateaus for all stage formations. This was supported by in-situ XRD analysis that shows diffraction signals from graphite intercalation compounds at more positive potentials in AR electrolyte. Further, electrolyte with the anion receptor was found to have significantly higher ionic conductivity at 00C, providing a way to design Li-ion battery electrolytes for low-temperature applications.
The second paper characterizes the electrolyte structure with focus on solvent/Li+, solvent/PF6¯ and Li+/PF6¯ interactions when anion receptor added to the electrolyte. It was found that the additive could reduce the concentration of ion pairs when added to EC, DEM and EC:DMC electrolytes. In-situ DEMS recorded during the first charge of the electrodes revealed the influence of the additive on the reduction patterns of the electrolyte components. C2H4 evolution was found to occur at more positive potential with narrower peak and significantly higher evolution rate, further confirming the changes in the SEI formation in the AR electrolyte. Similarly H2O reduction rate was found to be lower in AR electrolyte which was explained by its consumption through hydrolysis prior to reduction. Three distinct H2 evolution peaks were recorded and discussed in relation to EC reduction and Li+ intercalation onset. SEI chemistry on graphite anodes at various stages of the first discharge was also addressed via post-mortem DRIFT analysis and elaborated over a wide range of potentials from OCV to cut-off at 5 mV.
Third paper discusses the electrolyte structure and SEI chemistry in binary salt electrolyte systems (EC:DMC). NaPF6 was co-dissolved in a LiPF6 electrolyte at various concentrations such that the total molarity was always 1M. Reduction pattern and SEI chemistry were found to differ with increasing Na+ concentration. It was shown with FTIR analysis that NaPF6 addition resulted in increased DMC presence in Li solvation shell. Additional reduction plateau appeared at 1.7 V when Li+ and Na+ concentrations were both around 0.5M which was attributed to competition between the two cations for solvent (EC) solvation. Post mortem DRIFT analysis of the electrodes charged to 1.5 V revealed the presence of DMC reduction products when NaPF6 co-dissolved, further supporting the increased DMC in Li solvation shell. | nb_NO |
