Nanostructured architectures of silicon and germanium as high capacity, long cycle life lithium-ion battery electrodes

This thesis describes the development of high-capacity, next generation Li-ion battery (LIB) electrodes based on nanostructured Li-alloying materials, silicon and germanium. The results chapters are arranged as research articles with introductory summaries at the beginning of each. Boasting specific capacities that are several multiples of the standard anode material graphite (372 mAh/g), Si (3579 mAh/g) and Ge (1384 mAh/g) have immense potential as performance boosting replacement materials for future LIBs. However, a ~300 % volume expansion upon charging make bulk Si and Ge unsuitable for practical LIBs, due to associated pulverisation and delamination of the active material. The use of nanostructured architectures has mitigated the detrimental volume change issues, with stable cycling for the nanostructured active material previously reported. Both Si and Ge have respective benefits in LIBs and recently there has been a growing focus on combining Si and Ge within the same battery electrode. The use of a composite Si/Ge active material allows the high theoretical capacities and lower relative costs of Si and high conductivity and Li diffusivity (yielding promising rate capability performance) of Ge to be harnessed in a LIB. Chapter 3 describes the development of Sn seeded Si-Ge heterostructure nanowire (hNW) electrodes. The benefits of having Si and Ge within the same electrode is demonstrated, with high specific capacities of 1100 mAh/g being retained after 400 cycles and capacities of 613 mAh/g exhibited at 10C. An ex-situ microscopy study shows that the excellent battery performance of the electrode is enabled by a restructuring of the hNWs, from abrupt interfaces between segments (prior to lithium insertion) to a robust network of nanometre sized ligaments of an alloyed Si1-xGex composition. Chapter 4 describes the development of Ge/Si core/shell NWs through a secondary amorphous silicon (aSi) deposition onto as-grown Ge NWs. The deposition of Si layer, facilitated by an Expanding Thermal Plasma (ETP) technique enables the preparation of electrodes with increased mass loadings and high capacities, where capacities of up to 2066 mAh/g were achieved. This study investigated the electrochemical performance of these materials in both half-cell and full-cell configurations. As with the active material in chapter 3, the core/shell NWs converted into a Si1-xGex alloy due to repeated lithiation and delithiation. Chapter 5 details the synthesis of a Si1-xGex alloy NWs with four different compositions: Si0.67Ge0.33, Si0.50Si0.50, Si0.33Ge0.67 and Si0.20Ge0.80. It was found that the Si:Ge composition could be tuned by balancing the precursor ratios and reactivities that are simultaneously introduced to the reaction. The alloy NWs were grown directly on stainless-steel current collectors and electrochemically tested as anode materials. This study showed the benefits of being able to control the Si:Ge ratios, with the more Si-rich compositions exhibiting the highest capacities and the more Ge-rich compositions performing better at faster charge/discharge rates. The final two results chapters (6 and 7) concern the use of entirely Si based active materials for LIBs. In chapter 6, amorphous Si was deposited onto pre-formed Cu15Si4 NW arrays and used as LIB anodes for half-cell and full-cell testing. The Cu15Si4 NWs were synthesised from planar copper and operated as conductive, high surface area back-bones for Si deposition. Finally, Chapter 7 details the performance of Sn seeded Si NWs cycled in ionic liquid (IL) electrolytes. Here, the effect of electrolyte additives on the capacity retention of the material was studied and compared. The active material was cycled in a 0.1LiTFSI-0.6PYR13FSI-0.3PYR13TFSI IL as well as the IL with either EC or VC additive, where noticeable improvements for the additive containing tests was observed.