Improving the solid electrolyte interphase using pre-lithiation

This Thesis focuses on the use of pre-lithiation to influence properties of the solid electrolyte interphase (SEI) and it’s effects on the cycle life of the cells under investigation. Chapter 3 was the result of several experiments investigating the pre-lithiation of graphite. This work demonstrated that the material used for pre-lithiation could be different to the electrolyte and deliver the improved performance over simple pre-lithiation in the electrolyte. This opens the possibility for new pre-lithiation routes of electrodes that result in a lower resistance, longer lasting SEI than can be achieved with solely the electrolyte. Chapter 4 applies the same principles to Si based anodes. This found that whilst the initial capacity of these cells could be increased but the cells suffered from excessive capacity fade. This was found to be a result of SEI cracking/ reformation removing the benefits of a pre-lithiated SEI. Chapter 5 uses the information from chapter 4 to track the SEI reformation process using electrochemical impedance spectroscopy (EIS). This experiment found that the presence of FEC in the electrolyte lowered the resistance of the SEI, even when they were not pre-lithiated in their presence. The reverse was true when it was in the pre-lithiated SEI but not the electrolyte. Chapter 6 looked at pre-lithiated hard carbon electrodes as Na-ion anodes. It was found that pre-lithiated hard carbon anodes had a lower capacity as pre-lithiation time increased. The pre-lithiated electrodes had much greater capacity than the un-lithiated hard carbon which suffered from rapid capacity fade. The solubility of the pre-lithiated SEI was investigated, and it was found that allowing the cell to rest for extended periods increased the capacity of the anode with lower SEI resistance.