PROBING THE DEGRADATION MECHANISMS OF LI-ION CELLS CONTAINING SILICON AND SINGLE-WALLED CARBON NANOTUBES

Abstract

This work explores the improvements that can be made to energy density by changing the active materials within the negative electrode. Current batteries use graphite as the lithium-storing active anode material as it can reversibly incorporate lithium atoms efficiently and safely. Swapping graphite for a different kind of active material such as silicon presents many advantages but also many challenges surrounding lifetime. Silicon provides almost 10x the specific capacity of graphite and is incredibly abundant but experiences strong capacity fade due to its surface interaction with the electrolyte. The volume expansion silicon experiences during lithiation leads to constant parasitic chemical reactions and mechanical degradation leading to early cell failure. The introduction of a silicon anode in place of graphite improves stack energy density by up to 59%. The work done in this thesis explores alternative anode active materials including silicon and silicon based composite particles by evaluating their energy density and cycle life. Silicon active materials showed promising results when used in conjunction with single-walled carbon nanotubes and a conductive additive. The addition of carbon nanotubes to the electrode coating process allowed the use of common binders as it creates a strong electrical network between active particles that can withstand large volume changes. To further the development of silicon negative electrodes their degradation mechanisms and failure pathways in full cells must be fully understood. In-situ stack pressure, in-situ gas evolution and post cycling differential voltage analysis are used to pinpoint and track capacity loss mechanisms in full cells. These results highlight the failure of silicon based full cells as lithium inventory losses due to the constant SEI forming reactions on the anode. These results reinforce the need for engineered silicon composites that limit the particle volume fluctuations during cycling.