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Chapter 1
Introduction to Rechargeable LithiumāSulfur Batteries
Rezan Demir-Cakan
Department of Chemical Engineering,
Gebze Technical University, 41400 Gebze, Turkey
[email protected] 1.1.Introduction and History of LithiumāSulfur Batteries
1.1.1.Introduction
Rapid societal development and economic growth dictates the use of energy sources which are mostly based on fossil fuels. However, fossil fuels are neither continuous nor environmentally benign, leading to negative consequences in the environment caused by human energy needs. It is projected that the carbon dioxide (CO2) emissions will cause global warming resulting in a 4ā6Ā°C temperature raise by 2100.1 Likewise, fossil fuel depletion has been identified as a future challenge since it is foreseen that oil reserves will be terminated within the next 40 years while coal and natural gas may last at most for another 150 years.2 Hence, researchers are responsible for realizing new ideas on how to exploit renewable energy resources such as wind, water or sun in the most efficient manner without causing any further ecological calamities. However, most renewable energy sources are intermittent; thus, energy storage is one of the essential components of the forthcoming energy supply system to make use of renewable energy sources with fluctuating power output.3
Technological demands for higher capacity and cost-effective energy storage options provide an incentive for exploration of alternative electrochemical energy storage systems. Technologies that can provide economic growth as well as CO2 emission-free transportation by replacing the internal combustion engines with electric traction should be highlighted. Providing required flexible electricity generation and demand between daytime and night are highly important for grid operation.
New energy politics has led to the energy storage subject especially battery technologies becoming an important topic due to the development of mobile applications (i.e. electrical vehicles (EVs) or cellular phones). Over the past 25 years, Li-ion batteries (LIBs) played a crucial role in the development of such energy storage technologies. Today LIBs are used in 90% of rechargeable portable electronic devices. Although great improvements have been accomplished, and while active research for further developments continue, current Li-ion technologies provide a limited gravimetric energy density (100ā150 Wh/kg for a full system) which cannot compete with either fulfilling the goal of replacing combustion engines or meeting large mass energy storage backup dictated by solar farms or wind turbine plants. For instance, EVs powered by LIBs result in an inadequate driving range (160ā200 km) (Figure 1.1). Therefore, more drastic approaches are necessary to go beyond this limit and to accomplish āThe Holy Grailā of a 500 km driving range at low cost, without the need for hybridization with conventional internal combustion engines.
Earth abundant sulfur is one alternative to reach such goals. Lithiumāsulfur (LiāS) batteries offer around three fold increase in energy density compared with present Li-ion technologies. LiāS battery configuration operating at room temperature represents a valuable option as it can provide low equivalent weight, high capacity (1672 mAh/g), low cost (about $150 per ton) and environmentally benign factors. All these characteristics cannot be accomplished with current Li-ion technology.
Figure 1.1 Practical, specific energies for some rechargeable batteries, along with estimated driving distances.
Source: Reproduced with permission from Ref. 1.
1.1.2.History of LiāS batteries
The history of LiāS chemistry dates back to early 1960s (even prior to the advent of rechargeable Li batteries).4 With the patented work of Herbert and Ulam in 1962, sulfur was proposed as positive electrode and Li (or alloy of Li) as negative electrode in electric dry cells and storage batteries. Electrolyte was identified to be alkaline or alkaline-earth perchlorate, iodide, sulfocyanide, bromide, or chlorate dissolved in a primary, secondary or tertiary saturated aliphatic amine. Four years later, Herbert filed another patent,5 which was a continuation in part of their previous patent,4 with the electrolyte solution preferably consisting of a selected Li salt dissolved in a propyl, butyl or amyl amine. Preferably isopropyl amine was utilized as the solvent. In the same year, Rao6 patented high-energy density metalāsulfur batteries. Electrolyte consisted of cations of light metals or ammonium ions and anions of tetrafluoroborate, tetra-chloroaluminate, perchlorate or chloride salts which were dissolved in organic solvents. The solvents were propylene carbonate, y-butyrolactone, NzN-dirnethylformamide or dimethylsulfoxide and the cells were cycled between the voltages 2.52 and 1.16 V vs. Li. Later on, in 1970, Moss and Nole,7 represented a patent for the battery employing Li and sulfur electrodes with non-aqueous electrolyte. More information regarding the patent landscape of LiāS batteries, with analyzed and categorized total of 760 patent families, can be found in Ref. 8.
Although the concept of LiāS batteries is not new and was already intensively researched, the topic was inhibited because of missing exploitable results of the early studies.9 Following the pioneering work published by Nazar et al.10 in 2009, the topic was revisited and attracted drastic research interest in the field. The subsequent development of LiāS rechargeable batteries is enormous and has very high publication dynamic. The literature reports of research and review papers database from the Web of Science have shown results of over 2500 papers containing the phrase ālithium sulfur batteriesā (Figure 1.2(a)), papers with more than 70,000 citations, showing the importance of this field of research (Figure 1.2(b)).
A detailed analysis of the literature studies for LiāS batteries and their topic distribution can be seen in Figure 1.3. Most of the works were devoted to the design of host matrices for sulfur impregnation and formulation of cathode composite electrode (detailed information can be found in Chapter 2). Besides all these attempts to control polysulfide dissolution with the help of different cathode architectures, recently many efforts have been performed to find suitable and effective adsorption/absorption of polysulfides within the composite cathode.
Apart from these confinement strategies, some reports have proven that those polysulfides are beneficial for the cell life. Xu et al.11 have showed that the self-healing of LiāS batteries could be developed in the presence of polysulfide containing electrolyte by creating a dynamic equilibrium between the dissolution and precipitation of Li polysulfides at the electrode interfaces. Thus, research in the field of LiāS batteries is slightly moving from those sulfur confinements to the use of chemically synthesized dissolved polysulfides either in static condition or redox flow configuration.12ā14 Alternatively, those polysulfides are even employed as electrolyte additives for improved cycling performances.11,15,16 More information regarding the use of polysulfide, either as electroactive component or electrolyte additives can be found in Chapter 3.
An important progress during this period was the identification of the electrolyte formulation suitable for the LiāS batteries. Many solvent/salt combinations were suggested including, sulfone based...
