- As scientists, we have an affinity for order: repeating crystal structures, Euclidean space, continuous functions… Even concepts such as defects are discussed in the context of distracting from order. Things are most logical and best described in straightforward, taxonomical fashion. But what happens when the very application desired is dependent on the lack of order?
In many ways, the effective use of amorphous materials is the next frontier in material science. Whereas crystalline based materials, have very defined structures and properties, amorphous materials are not beset but such rigidities. Due to the wide range of possibilities in structure, properties, and complexities, they open up a whole new world of applications.
However, for all the promises offered by amorphous materials, they also come with a significant amount of challenges: synthesis, characterization, structure-property relationships, and repeatability all come to mind. Furthermore, the very nature of the word, amorphous coming from the Greek "amorphe: symbolizing "without shape", indicates there is no clearly defined way of classifying the structure. Different versions of the same amorphous materials lead to different properties, yet they all earn the same designation, as there are no straightforward ways of telling them apart. In a way, these are very much a double-edge sword: the very thing that makes them useful, is also what makes them difficult to work with -- and thus the reason you assign them to a graduate student!
As such, the following work deals with two seemingly disparate issues, yet very much inextricably linked: amorphous carbon and Na-ion batteries (NIBs). More specifically, the two are connected due to the role of amorphous carbon as an NIB anode material.
The NIB topic was of particular interest, as there currently exists an unmet need for scalable electrochemical energy storage systems. Current solutions in this domain are either too expensive due to material costs, or do not meet to requisite performance requirements to be an effective option. But, NIBs, with their low overall material costs, and adequate electrochemical performances offer a path forward.
However, there still remains underlying challenges before commercially viable NIBs become a reality. One of the key concerns is the state of the negatively charged electrode -- anode for short -- material. Typical alkali ion batteries such as Li-ion batteries (LIBs) or K-ion batteries (KIBs), use graphite as an anode material due to its ease of forming reversible graphite intercalation compounds (GICs). Unfortunately, due to thermodynamic constraints, Na atoms are not afforded the same possibilities. They cannot form reversible GICs, let alone form GICs to begin with. Thus for this reason, other candidate anode materials must be explored.
To that end, amorphous carbon is one of the most promising. The wider spacing and presence of defects in the structure eliminate much of the spatial and thermodynamic constraints that beset the use of graphite. Additionally, the material is cheap, abundant, easy to synthesize -- at least on the lab-scale -- and non-toxic, which makes it an ideal candidate for further exploration.
Herein, this dissertation addresses two main topics. The first one focuses on the electrochemistry, and structure property relationships linking amorphous carbon and NIBs. In that work, issues such as surface area, porosity, Na-atom storage mechanism, and electrochemical performance are discussed. This newly discovered information is of interest to the battery community, as many of its discoveries can be used to build better batteries, and battery materials proceeding forward.
The second topic is that of amorphous carbon itself, and discoveries on its synthesis process, as well as novel ways of exploring its structure and characterizing it. As stated before, one of the key challenges in working with amorphous materials, is to find unbiased and straightforward methods of characterizing them. In a way, one has to make order from chaos. Thus, new models and approaches to doing this are discussed, which should be a topic of interest to any researcher that primarily works with amorphous materials.
The subsequent work presented was conducted in the years ranging from 2013 to 2017, and was carried out using both experimental methods, as well as computational/numerical methods to test some of the theories that were proposed in the experimental work. It is the intention of the author that those who should complete the reading of this manuscript should come away with a better understanding of amorphous carbon, its characterization, and how it pertains to NIBs.