Nano-structuring of silicon and germanium :a viable route to high capacity, long cycle life lithium-ion battery electrodes

This thesis describes the development of high-capacity, next generation Li-ion battery electrodes based on germanium and silicon nanostructures grown directly from stainless steel current collectors. The core chapters are arranged as research articles with introductory summaries at the beginning of each. The anode component of current commercial Li-ion batteries is typically composed of graphite (theoretical capacity of 372 mAhg-1); even though, Ge and Si anodes boast multiples of this due to their ability to form lithium rich alloys. However, the formation of these high capacity lithiated alloys, Li15Ge4 (1384 mAhg-1) and Li15Si4 (3579 mAhg-1), leads to considerable expansion of bulk Ge and Si electrodes (> 300%) which causes pulverisation of the material and loss of contact with the current collector, ultimately limiting the cycle life of Li-alloying anodes. Nanowire based electrodes overcome this as they circumvent the pulverisation issue due to the unique properties bestowed upon them by their nano-dimensions. Chapter 3 describes the development of Sn seeded Ge NW electrodes that retain very high capacities of 900 mahg-1 after 1100 cycles and also display excellent rate performance characteristics. Conventional wisdom in the field is that nanowire based materials outperform their bulk counterparts as the smaller dimensions enable the material to retain their wire shape and resist deformation despite the large volume changes occurring. However, through an ex-situ electron microscopy study, it is shown here that this is not the case and the excellent performance of the electrode can in fact be attributed to a complete restructuring of the active material that occurs within the first 100 cycles, to form a continuous, porous, mechanically robust network of germanium ligaments. Chapter 4 describes the development of a simplified, scalable, solvent free method, rapid pyrolysis approach to fabricate germanium nanowire based Li-ion anodes, which again show excellent capacity retention and rate capability over extended cycles. Chapter 5 describes the synthesis of Ge/Si composite nanowire electrodes, wherein Ge nanowires are grown with Si nanowire branches extending from them. When utilised as Li-ion anodes, the material combines the stability and high rate capability of Ge with the higher capacity of Si. Finally, an investigation into the efficacy of nanowire electrodes based solely on Si is presented in chapter 6. The effect of electrolyte composition on the capacity retention of the material, and the compositional and electrochemical properties of solid electrolyte interface layer were investigated.