High Performance Germanium-based Anode Materials for Lithium-ion and Sodium-ion Rechargeable Batteries

  • Author / Creator
    Farbod, Behdokht
  • In this thesis the electrochemical performance of germanium nanowires (GeNWs) as anode for lithium-ion batteries (LIBs) and tin-germanium-antimony (Sn-Ge-Sb) thin films as anode for sodium-ion batteries (NIBs) have been investigated. Scientific literature shows a substantial study-to-study variation in the electrochemical lithiation performance of "1-D" nanomaterials such as Si and Ge nanowires or nanotubes. In chapter 2 of this thesis, we varied the vapor-liquid-solid (VLS) growth temperature and time, resulting in nanowire arrays with distinct mass loadings, mean diameters and lengths, and thicknesses of the parasitic Ge films that are formed at the base of the array during growth. When all the results were compared, a key empirical trend to emerge was that increasing active material mass loading drastically degraded the electrochemical performance. For instance, GeNWs grown for 2 minutes at 320 °C (0.12 mg cm-2 mass loading, 34 nm mean nanowire diameter, 170 nm parasitic film thickness) had a reversible capacity of 1405 mAh g-1, a cycle 50 coulombic efficiency (CE) of 99.9%, a cycle 100 capacity retention of 98%, and delivered ~ 1200 mAh g-1 at 5C. To contrast, electrodes grown for 10 minutes at 360°C (0.86 mg cm-2, 115 nm, 1410 nm) retained merely 5.6% of their initial capacity after 100 cycles, had a CE of 96%, and delivered ~ 400 mAh g-1 at 5C. Using TOF-SIMS we are the first to demonstrate marked segregation of Li to the current collector interface in planar Ge films that are 300 and 500 nm thick, but not in the 150 nm specimens. FIB analysis shows that the cycled higher mass loaded electrodes develop more SEI and interfacial cracks near the current collector. A comparison with the state-of-the-art scientific literature for a range of Ge - based nanostructures shows that our low mass loaded GeNWs are highly favorable in terms of the reversible capacity at cycle 1 and cycle 100, steady-state cycling CE and high-rate capability. Chapter 3 provides the first report on several compositions of ternary Sn-Ge-Sb thin film alloys for application as sodium ion battery (aka NIB, NaB or SIB) anodes, employing Sn50Ge50, Sb50Ge50 and pure Sn, Ge, Sb as baselines. Sn33Ge33Sb33, Sn50Ge25Sb25, Sn60Ge20Sb20 and Sn50Ge50 all demonstrate promising electrochemical behavior, with Sn50Ge25Sb25 being the best overall. This alloy has an initial reversible specific capacity of 833 mAhg-1 (at 85 mAg-1), and 662 mAhg-1 after 50 charge - discharge cycles. Sn50Ge25Sb25 also shows excellent rate capability, displaying a stable capacity of 381 mAhg-1 at a current density of 8500 mAg-1 (~ 10C). A survey of published literature indicates that 833 mAhg-1 is among the highest reversible capacities reported for a Sn-based NIB anode, while 381 mAhg-1 represents the most optimum fast charge value. HRTEM shows that Sn50Ge25Sb25 is a composite of 10 - 15 nm Sn and Sn-alloyed Ge nanocrystallites that are densely dispersed within an amorphous matrix that also contains localized "buffering" nanoporosity. Comparing the microstructures of alloys where the capacity significantly exceeds the rule of mixtures prediction to those where it does not, leads us to hypothesize that this new phenomena originates from the Ge(Sn) that is able to sodiate beyond the 1:1 Na:Ge ratio reported for the pure element. Combined TOF-SIMS, EELS TEM and FIB analysis demonstrates substantial Na segregation within the film near the current collector interface that is present as early as the second discharge, followed by cycling - induced delamination from the current collector.

  • Subjects / Keywords
  • Graduation date
    Fall 2014
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.