In 2014, a team from industry and academia came together to develop a Revolutionary Electric Vehicle Battery and Energy Management System (REVB) funded by the EPSRC and Innovate UK. The project brought together material scientists, electrochemists, physicists, mathematicians, and engineers from OXIS Energy Ltd, Imperial College London, Cranfield University, Lotus Engineering, and Ricardo PLC. In 2016, the team held the first lithium–sulfur conference in the United Kingdom in the Faraday lecture theatre of the Royal Institution in London – a place where scientists, artists, authors, and politicians have shared ideas for over 200 years with the aim of diffusing science for the common purpose of life. The conference was named LiS–M³ and continues annually. M³ stood for materials, mechanisms, and modeling; we also held a fourth session for applications of Li–S technology. Following that first conference, the two conference chairs, Dr. Gregory J. Offer from Imperial College and Dr. Mark Wild from OXIS Energy, were approached to edit this book. The chapters have been provided by those that gave presentations, were invited that day, or were part of the REVB team in the Faraday lecture theatre.

The organization of this book follows the structure of the conference and has the same aim of educating a diverse scientific community about the most promising next‐generation Li–S battery technology, enabling applications requiring batteries with superior gravimetric energy density. Lithium–sulfur batteries are game changers in the world of lightweight energy storage with a theoretical gravimetric energy density of ∼2600 Wh kg⁻¹. Yet, there are challenges, and today the practical energy density target is 500 Wh kg⁻¹.

• Materials . In Part I we start with basic electrochemical theory to understand the challenges and complexity of lithium–sulfur batteries, and then focus on the approaches by material scientists to overcome those challenges. It soon becomes clear that there are no silver bullets, but that a systems approach is required to increase the areal loading of sulfur in a stable cathode, to increase sulfur utilization through the electrolyte/cathode interface and to reduce degradation at the electrolyte/anode interface. It is also evident that lithium–sulfur technology has reached a point in its development that it can now be tailored to meet the needs of commercial markets such as aviation, marine, or automotive.
• Mechanisms . Part II considers the current understanding of the complex mechanisms in a lithium–sulfur cell. There remains an incomplete understanding of the mechanism from materials research, and analytical studies only see part of the picture. Elucidating the mechanism of a lithium–sulfur cell is complex and intriguing. There are many studies that have opened windows onto the association and disassociation reactions of the lithium polysulfides and the precipitation and dissolution of solid products at the end of charge and discharge. These underlying mechanisms lead to the unique discharge and charge characteristics and degradation pathways. Included are chapters on polysulfide reactivity and an enlightening look at the lithium–sulfur cell from the perspective of its insoluble end product, lithium sulfide.
• Modeling . Part III starts by looking broadly at physics‐based models that mimic and predict the performance and degradation of a lithium–sulfur cell under operational conditions. The section concludes with control models used to predict state of charge and state of health in real‐time battery management systems. Modeling requires knowledge of the mechanism (Part II) and performance characteristics of the technology (Part I) and is used to develop the control algorithms and working models required by engineers developing applications (Part IV).
• Applications . Part IV addresses the commercial application of lithium–sulfur battery technology. It starts with a market analysis, takes in key differences that battery engineers must be aware of in the design of a lithium–sulfur battery, and concludes with the first real‐world application of lithium–sulfur batteries, the high‐altitude long‐endurance unmanned aerial vehicle (HALE–UAV).

As a guide to access the book, each part begins with its own introduction and also each chapter. If you are looking for a good overview, then start with the part introductions. If you are a material scientist, then start with Part I and continue with mechanisms in Part II; Chapter 10 may also be of interest to identify a target market. If you are an applications engineer you might like to start with modeling in Part III and move to applications in Part IV. If you are interested in modeling lithium–sulfur cells then start with Chapter 2 of Part I and then move to Parts II and III.

Lithium–sulfur cells utilize a very similar architecture as today's Li ion pouch cells. Double side coated cathodes (sulfur) are assembled with layers of separators and anodes (lithium foil) either through winding or electrode stacking and subsequently vacuum packaged into a pouch (aluminum/polymer laminate foil). In Li–S cells, cathodes are typically assembled in the charged state with lithium metal as anode.

At first glance, the combination of the lightest, most electropositive metal (lithium) with a safe, abundant (and reasonably light) nonmetal (sulfur) makes good sense as a prospective battery. However, while the lithium–sulfur battery offers a very high theoretical specific energy (∼2600 Wh kg⁻¹) the actual performance delivered is proving to be limited and today a gravimetric energy density target of 500 Wh kg⁻¹ is thought to be an achievable step change in battery performance with this technology.

Materials research lies at the heart of lithium–sulfur cell development and relies on a good understanding of the underlying mechanisms (Part II). The game changer is to achieve a lightweight battery with sufficient cycle life and power performance for relevant applications (Part IV) where the weight of large Li ion battery systems hinders product performance, e.g. aircraft and large vehicles. The goal is to increase the ratio of active sulfur to inactive, yet functional, materials in the cell and to make the best use of this sulfur by achieving the highest sulfur utilization cycle on cycle.

In Chapter 1, we begin with a grounding in basic electrochemical theory. We explore how basic theory translates to a more complex electrochemical system such as a lithium–sulfur cell. The chapter concludes with a theoretical explanation of the main challenges faced by materials research scientists developing commercial lithium–sulfur products.

In Chapter 2, we move on to a discussion of the sulfur cathode, where due to its nonconductive nature, sulfur is most often combined with carbons, additives, and binders to be coated onto a primed aluminum current collector. Even when optimized for high areal sulfur loading, cathode materials contribute to reduced gravimetric energy density and release reactive polysulfides into the electrolyte, leading to degradation.

In Chapter 3, we continue with a discussion of electrolytes and it will become apparent that there is an intimate relationship between sulfur loading and electrolyte loading. Stability of the electrolyte components toward both lithium and polysulfides is also critical to optimizing sulfur utilization and cycle life. A balance is to be struck between trapping polysulfides within the cathode and dissolution of polysulfides into the electrolyte to achieve acceptable energy density, cycle life, and power.

In Chapter 4, we briefly summarize the electrolyte anode interface and the key challenges. This is an area that is poorly covered by the academic literature but is a vital area of research to improve the cycle life of a lithium sulfur cell in tandem with other approaches. Throughout all chapters and in Chapter 7 we make reference to the role of anode in relation to degradation and reduced cycle life. Primarily efforts have included the use of a range of barrier layers either at the cathode surface, as a modification to the separator or as a polymer, or at the ceramic coating on the lithium itself in addition to optimization of cathode and electrolyte formulations.

Not to be lost is the shift in lithium–sulfur cell development to choose the thinnest and lightest components to reduce gravimetric energy density. Such materials require special consideration during scale‐up activities when compared to handling procedures in standard lithium ion manufacture.