The PhD evaluation committee was:

• Professor Erik Berg, Uppsala University, Sweden
• Dr Sigita Trabesinger, Paul Scherrer Institute, Switzerland
• Professor Andreas Erbe, Department of Materials Science and Engineering, NTNU, Norway

The doctoral work has been carried out at the Department of Materials Science and Engineering, where Professor Ann Mari Svensson has been the candidate’s supervisor and Professor Mari-Ann Einarsrud and Fride Vullum-Bruer has been the candidate’s co-supervisor.  

Daniel Tevik Rogstad is currently employed as a Senior Engineer at Freyr Battery.

Summary:

In this work, research is focused on the use of silicon as anode material for Li-ion batteries, combined with thermally stable and safe electrolytes. The silicon of choice is micrometer sized metallurgical silicon (µMG-Si). µMG-Si is interesting mainly due to its cost advantage over different forms of nanostructured high-purity silicon. The low surface area of this material, and hence the reduced SEI formation and repair, can potentially improve the cyclability. The silicon anodes are studied in combination with electrolytes based on the salt lithium bis(fluorosulfonyl)imide (LiFSI) dissolved in various ionic liquids (ILs), expected to improve the overall safety of the battery due to ILs inherent nonvolatility, and enable higher voltage cathodes due to higher oxidative stability. In the first part of the thesis, an in-depth study of four promising ionic liquids combined with µMG-Si anodes was conducted, to compare conductivity, electrochemical stability, electrochemical performance and wetting properties combined with post mortem analysis of cross sections and the surface (SEI layer). The ionic liquids of choice were based on pyrrolidinium (PYR13 + ), imidazolium (EMI+) and phosphonium (P111i4+) cations, all combined with the FSI anion, and in addition the IL PYR13bis(trifluorosulfonyl)imide (PYR13TFSI). While the FSI based ionic liquids are known to possess low viscosity and good conductivity, the TFSI based ones are typically more thermally and electrochemically stable. Cells with the PYR13FSI and P111i4FSI electrolytes showed the best cycling stability, maintaining reversible capacities of over 1200 mAh/gSi at a rate of C/5 (cycle 100) and over 2000 mAh/gSi at a rate of C/20 (cycle 101), as well as the highest average coulombic efficiency. The poor cycling stability of a standard carbonate electrolyte, and also the EMIFSI ionic liquid, was attributed to the higher degree of lithiation in the initial cycles and a poorer SEI, and correspondingly faster degradation. The initial lithiation was affected by both the Li-ion mobilities and the wetting of the solid electrode surface by the electrolyte. For the ionic liquids, the capacitance of the electrodes increased significantly after cycling, indicating an increase in electrochemically active surface area. The opposite was observed for the carbonate electrolyte. For electrodes cycled in the FSI based ionic liquids, the SEI appeared to be composed of mainly salt reduction products. At higher rates, the EMIFSI ionic liquid achieved the highest reversible capacities of the IL electrolytes, retaining a discharge capacity of >500 mAh/gSi at a rate of 2C. Cells with the PYR13FSI and P111i4FSI electrolyte had only a limited capacity at a rate of C/2 and higher. Overall, LiFSI:PYR13FSI and LiFSI:P111i4FSI are the most viable candidates as safer electrolytes for µMG-Si anodes.

As all the ionic liquids suffered from poor rate performance, two strategies for improvements were investigated. The first one was simply to increase the temperature, and during cycling at 60 ◦C, two of the electrolytes, LiFSI:P111i4FSI and LiFSI:EMIFSI actually outperformed the conventional carbonate electrolyte, with the best rate performance observed for LiFSI:P111i4FSI. This is partly attributed to a higher increase in conductivity for these electrolytes, and in particular the higher Li-ion conductivity. In addition, we propose that the excellent performance of LiFSI:P111i4FSI is related to the good SEI forming properties for this electrolyte. For operation at 60 °C, however, the cycle life of the batteries will be compromised, an additional challenge that needs to be solved. The second strategy for improving rate performance was to use mixtures with one selected ionic liquid and carbonate co-solvents and/or additives. Two mixtures were investigated, namely LiFSI dissolved in PYR13FSI mixed with either the carbonate solvent dimethyl carbonate (DMC), or a combination of fluoroethylene carbonate (FEC) and DMC. It was verified that the electrolytes composed of LiFSI in PYR13FSI:DMC (20:40:40 mol%) and LiFSI in PYR13FSI:DMC:FEC (20:40:30:10 mol%) were thermally stable up to 100 °C. Both mixtures showed an improved rate performance, although not reaching the rate performance of the conventional carbonate electrolyte at room temperature. However, a reasonable cycling stability could only be obtained upon
addition of FEC.