Author ORCID Identifier

http://orcid.org/0000-0002-2736-1733

Year of Publication

2016

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Engineering

Department

Chemical and Materials Engineering

First Advisor

Dr. Yang-Tse Cheng

Abstract

Silicon is capable of delivering a high theoretical specific capacity of 3579 mAh g-1, which is about 10 times higher than that of the state-of-the-art graphite-based negative electrodes for lithium-ion batteries (LIBs). However, the poor cycle life of silicon electrodes, caused by the large volumetric strain during cycling, limits the commercialization of silicon electrodes. As one of the essential components, the polymeric binder is critical to the performance and durability of lithium-ion batteries as it keeps the integrity of electrodes, maintains conductive path and must be stable in the electrolyte. The guideline for binder selection of silicon electrodes is still not available as the electrochemical performance of silicon is very challenging and lots of research are still being carried out.

This dissertation is focused on unveiling the critical role of polymeric binders in silicon negative electrodes. As a first step, silicon electrodes mixed with commercially available Nafion and ion-exchanged Nafion were demonstrated to maintain a high specific capacity over 2000 mAh g-1 cycled between 1.0 V and 0.01 V, compared with the traditional binder polyvinylidene fluoride (PVDF). Stable cycling at 1C rate for more than 500 cycles was achieved by limiting the lithiation capacity to 1200 mAh g-1.

Secondly, a comprehensive study of the binding mechanisms of these binders in silicon/LiNi1/3Mn1/3Co1/3O2 full cells was carried out by using techniques such as X-ray photoelectron spectroscopy (XPS) and instrumented nanoindentation. Partial charge/discharge testing with controlled silicon lithiation capacity showed that the ion-exchanged Nafion and sodium alginate were both effective binders to maintain 1200 mAh g-1 for a long period of cycling without capacity decrease. Full charge/discharge testing showed that the ion-exchanged Nafion and sodium alginate binders exhibited the highest capacity retention when the volume change of silicon nanoparticles was about 300%.The superior performance of ion-exchanged Nafion was due to its capability to conduct Li+ to isolated silicon nanoparticles. Binders would not affect the composition of solid electrolyte interphase (SEI). Therefore, coupled chemical degradation (SEI growth, lithium consumption) and mechanical degradation (cracking, particles isolation) are the cause of the failure of the full cells.

Digital Object Identifier (DOI)

https://doi.org/10.13023/ETD.2016.437

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