Silicon and Germanium Nanowires Anode Materials for Lithium and Sodium-ion Batteries

• Author / Creator

  Kohandehghan, Alireza

• This thesis is focused on the silicon-based anode materials for lithium-ion batteries (LIBs) as well as germanium-based electrode materials for sodium-ion batteries (NIBs). In our first attempt we studied electrochemical cycling stability and degradation mechanisms of silicon nanowires (SiNWs) coated with Mg and Mg2Si for LIB anodes. Compared to SiNWs, both Mg-and Mg2Si-coated materials show significant improvement in coulombic efficiency (CE) during cycling, with pure Mg coating being slightly superior by ~ 1% in each cycle. XPS measurements on cycled nanowire forests showed lower Li2CO3 and higher polyethylene oxide content for coated nanowires, thus revealing a passivating effect towards electrolyte decomposition. The formation of large voids between the nanowire assembly and the substrate during cycling, causing the nanowires to lose electrical contact with the substrate, is identified as an important degradation mechanism. In our second attempt we demonstrated that nanometer-scale TiN coatings deposited by atomic layer deposition (ALD), and to a lesser extent by magnetron sputtering, will significantly improve the electrochemical cycling performance of SiNWs LIB anodes. A 5 nm thick ALD coating resulted in optimum cycling capacity retention (55% vs. 30% for SiNWs, after 100 cycles) and CE (98% vs. 95%, at 50 cycles), also more than doubling the high rate capacity retention (e.g. 740 vs. 330 mAh/g at 5C). The conformal 5 nm TiN remains sufficiently intact to limit the growth of the solid electrolyte interphase (SEI), which in turn both improves the overall CE and reduces the life-ending delamination of the nanowire assemblies from the underlying current collector. Our third attempt was demonstrating cycling performance improvement for SiNWs LIB anodes by a thin partially dewetted coating of Sn. The optimum architecture 3Sn/SiNWs (i.e. a Sn layer with an average film thickness of a 3 nm covering the nanowire) maintained a reversible capacity of 1865 mAh/g after 100 cycles at a rate of 0.1C. This is almost double of the SiNWs, where the reversible capacity after 100 cycles was 1046 mAh/g (~ 78% improvement). The 1Sn/SiNWs and 3Sn/SiNWs electrodes demonstrated much improved cycling CE, with > 99% vs. 94 - 98% for SiNWs. At a high current density of 5C, these nanocomposite offered 2X the capacity retention of bare SiNWs (~ 20 vs. ~ 10% of 0.1C capacity). It is demonstrated that the Sn coating both lithiates and delithiates at a higher voltage than Si and thus imparts a compressive stress around the nanowires. This confines their radial expansion in favor of longitudinal, and reduces the well-known failure mode by lithiation-induced nanowire stranding and fracture. TOF-SIMS analysis on the post-cycled delithiated specimens shows enhanced Li signal near the current collector due to accelerated SEI formation at the interface. FIB demonstrates concurrent en-masse delamination of SEI agglomerated sections of the nanowires from the current collector. Both of these deleterious effects are lessened by the presence of the Sn coatings. Germanium is a promising sodium ion battery (NIB, NAB, SIB) anode material that is held back by its extremely sluggish kinetics and poor cyclability. In our last attempt we demonstrated for the first time that activation by a single lithiation - delithiation cycle leads to a dramatic improvement in practically achievable capacity, in rate capability and in cycling stability of Ge nanowires (GeNWs) and Ge thin films (GeTF). TEM and TOF-SIMS analysis shows that without activation, the initially single crystal GeNWs are effectively Na inactive, while the 100 nm amorphous GeTF sodiate to only less than half their thickness. Activation with Li induces amorphization (in GeNWs) reducing the barrier for nucleation of the NaxGe phase(s), while introducing a dense distribution of nanopores that reduce the Na solid-state diffusion distances and buffer the sodiation stresses. The resultant sodiation kinetics are promising: Tested at 0.15C (1C = 369 mA/g, i.e. Na:Ge 1:1) for 50 cycles the GeNWs and GeTFs maintain a reversible (desodiation) capacity of 346 mAh/g and 418 mAh/g. The nanowires and films demonstrate a capacity of 355 and 360 mAh/g at 1C and 284 and 310 mAh/g at 4C, respectively. Even at a very high rate of 10C the GeTF delivers over 169 mAh/g.

• Subjects / Keywords

  • Lithium ion batteries
  • Sodium ion batteries
  • Silicon
  • Germanium
  • Nanowires

• Graduation date

  Spring 2015

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3G44HZ8Q

• 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.