| dc.description.abstract | Li-ion batteries have become the superior rechargeable battery technology due to high energy density, long cycle life, safety and cost. Several materials can be used for the electrodes and the electrolyte, enabling customization of battery properties to applications. Silicon is an interesting anode material because of higher capacity compared to graphite. The drawback with silicon is the volume change during operation, which imposes large strains on the electrode and leads to early failure. One way to limit the consequences of the volume change is having a well-functioning solid electrolyte interphase (SEI) on the anode. It has been suggested that lithium bis(fluorosulfonyl)imide (LiFSI) has better SEI forming properties than the commercially used electrolyte salt, lithium hexafluorophosphate (LiPF6). This study investigates LiFSI as a possible electrolyte salt for silicon-based Li-ion batteries.
The first part of this work compares the electrochemical performance of high content silicon electrodes in electrolyte containing LiPF6 or LiFSI salt. Higher performance was achieved in LiFSI electrolyte, and the reason was attributed to differences in the SEI on silicon. The SEI was investigated post mortem using transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The SEI formed in LiFSI electrolyte was homogeneous, more flexible, and had a lower resistivity than the SEI formed in LiPF6 electrolyte.
Secondly, the effect of increasing the LiFSI concentration on the performance of high content Si electrodes and NMC cathodes in half cell configuration was investigated. For the Si electrodes, 5M LiFSI electrolyte was found to have excellent rate capability. However, for long term cycling, none of the highly concentrated electrolytes could compete with the 1M LiFSI electrolyte. SEI characterization indicated that in highly concentrated LiFSI electrolyte SEI, was less stable than in 1M LiFSI electrolyte. For NMC cathodes, the highly concentrated LiFSI electrolytes resulted in improved performance compared to 1M LiFSI electrolyte. However, none of the LiFSI electrolytes could compete with 1M LiPF6.
Lastly, full cells with high content silicon anodes and NMC cathodes were investigated in electrolytes with pure LiPF6 and LiFSI, and LiFSI electrolyte with lithium difluoro(oxolate)borate (LiDFOB) added. The latter slightly improved performance compared to the other two. In the pure LiFSI electrolyte, no side reactions were observed, and the performance was comparable to LiPF6 electrolyte. Hence, LiFSI can be used as electrolyte salt in full cells with Si anode and NMC cathode. XPS analysis indicated that the SEI formed with LiFSI was more stable during cycling than the SEI formed with LiPF6. Half cell experiments with NMC cathodes and the two LiFSI electrolytes revealed that at a cut-off voltage of 4.5 V, addition of LiDFOB is needed to ensure stable cycling in a LiFSI-based electrolyte. | en_US |
