In-situ Investigations of Lithium-Sulfur Batteries

Licentiate thesis, 2019

As the demand for high energy-density storage devices increases, we must look beyond the current state of the art technology, the lithium-ion battery. Lithium-ion battery technologies are approaching their theoretical limit in terms of capacity, and now that the demand for longer-range electric vehicles (EVs) and the implementation of grid storage is increasing, we need to provide technologies that can go beyond what is currently possible. In order to increase the capacity of batteries, and to develop more sustainable technologies to meet the rising demand, we must turn to new chemistries.

A suggested Next-Generation Battery chemistry is based on the electrochemical reaction between lithium and sulfur. This chemistry does not rely on intercalation reactions as the Li-ion battery is, but instead employs conversion chemistry. At discharge elemental sulfur is reduced and converted to polysulfides, yielding a maximum specific capacity of 1672 mAhg-1, up to 6 times the theoretical maximum capacity of state-of-the-art Li-ion battery materials. Thus, the lithium-sulfur technology is a suitable successor due to a potentially higher energy density. In addition, there is also the potential to create sustainable systems made from low-cost and high abundance elements, while also creating less toxic and safer devices than those which are currently available for commercially.

In our quest to reach a working lithium-sulfur battery there are a series of challenges that must be addressed, many of which originate from the complex reactions and mechanisms of the lithium-sulfur cell. Soluble Li-polysulfide species are formed during cell operation in commonly used electrolytes, these species are highly mobile and react with the Li-metal anode used. This interaction leads to the unwanted reduction of polysulfide species at the anode, causing the polysulfide shuttle, and capacity fade due to the irreversible deposition of active material on the Li-metal surface.

A series of methods have been used to address the unwanted reactions, such as the use of novel additives in the electrolyte to form a stable solid-electrolyte interphase (SEI). In this thesis the unique character of polysulfide species is addressed, and methods discussed will show how control of polysulfide dissolution and speciation can be used to improve cell performance. This improvement is realised by designing new electrolytes that block the passage of polysulfides to the Li-metal anode’s surface, and by using polysulfide species in the electrolyte to enable longer lifetime cells by preventing sulfur dissolution while simultaneously supplementing the energy density of a cell by acting as a Li-salt. However, the mechanism of how the polysulfide species behave is not fully understood. To monitor how polysulfides interact with the Li-metal when they act as charge carriers, operando Raman spectroscopy has been employed to track polysulfide concentration changes in a cell and reveal new insights on the mechanisms of polysulfides as Li-salts.