Oxis Energy (United Kingdom)

As we approach the limits of lithium-ion batteries, new alternative chemistries are required[1]. Those that involve the sulfur conversion reaction at the positive electrode are among the most promising to move beyond Li-ion[2]. The conversion reaction of sulfur to form lithium sulfide is characterized by a high theoretical specific capacity of 1672mAh g-1, translating in a theoretical energy density of c.a. 2500 Wh kg-1, a value 2.5 times higher than that of commercial state-of-the-art Li-ion cells, using transition metal oxide based cathodes[3]. There are well-documented challenges associated with the traditional Li-S system which have been demonstrated to restrict the practical energy density and the cycle life[4]. Soluble polysulfides are generally formed, resulting in the shuttling mechanism, a major issue causing low coulombic efficiency, high self-discharge rates and negative electrode corrosion[4]. This reaction is also associated with large volume changes. This can lead to cell failure, due to the alteration of the cathode and anode structure and the favoured formation of lithium dendrites[5]. Strategies to enhance cycle life, while maximizing practical energy density, require a holistic approach. This includes optimization of the positive electrode (e.g., selection of conversion reaction catalyst, use of polysulfide adsorbents, binder selection); the negative electrode (e.g., lithium coating, functionalization and protection); the separator (e.g. functional barriers and polymeric membranes); the electrolyte (e.g. use of alternative solvents and salts, additives). This work will focus on the recent negative electrode advances at OXIS Energy Ltd, the pioneer in the research and development of large-scale, high-energy, longer cycle life lithium-sulfur batteries. [1] D. A. J. R. Ronald M. Dell, Lithium Batteries, John Wiley & Sons, Inc., Hoboken, NJ, USA, 2013. [2] B. Scrosati, J. Hassoun, Y.-K. Sun, Energy Environ. Sci. 2011, 4, 3287. [3] M. S. Whittingham, Proc. IEEE 2012, 100, 1518. [4] A. Manthiram, Y. Fu, S. H. Chung, C. Zu, Y. S. Su, Chem. Rev. 2014, 114, 11751. [5] X. Liu, J.-Q. Huang, Q. Zhang, L. Mai, Adv. Mater. 2017, 29, 1601759.

At a fundamental level, the operation of a lithium–sulfur (LiS) cell depends on many elementary physical processes occurring over a wide range of length scales: a large number of electrochemical reactions occur both sequentially and concurrently to maintain a flow of current between the cell's electrodes, but the overall performance depends on how these reactions are affected by intermediate chemical reaction steps, species transport, and the state of the cell; for example, its temperature and the morphology of its components. Although all battery chemistries depend on multiple processes occurring simultaneously during operation, the LiS cell is more complex than most due to its large number of solution-phase active species and multiple instances of each process type, often occurring in parallel. From this perspective, understanding how a LiS cell behaves is the study of how complex reaction–diffusion systems behave. The intent of this chapter is to give an overview of the elementary processes, how they interact, and how they can be used to build up a picture of what is occurring inside the cell during its operation. Because the processes themselves are generic, much of the subject matter is applicable to electrochemical cells in general, although examples are drawn from LiS technology where appropriate. Section 1.1 provides a brief top-level view of a LiS cell and introduces some of the terminology used in the subsequent discussion; Section 1.2 looks in more detail at the electrochemical processes and how these lead to a voltage being measured in a cell; Section 1.3 considers the behavior when a current is drawn from the cell; and Section 1.4 looks more specifically at a LiS cell and how particular processes affect the electrochemical behavior.

Lithium Sulfur (Li-S) battery is generally considered as a promising technology where high energy density is required at different applications. Over the past decade, there has been an ever increasing volume of Li-S academic research spanning materials development, fundamental understanding and modelling, and application-based control algorithm development. In this study, the Li-S battery technology, its advantages and limitations from the fundamental perspective are firstly discussed. In the second part of this study, state-of-the-art Li-S cell modelling and state estimation techniques are reviewed with a focus on practical applications. The existing studies on Li-S cell equivalent-circuit-network modelling and state estimation techniques are then discussed. A number of challenges in control of Li-S battery are also explained such as the flat open-circuit-voltage curve and high sensitivity of Li-S cell’s behavior to temperature variation. In the last part of this study, current and future applications of Li-S battery are mentioned.

Recent research into lithium ion battery storage particles has seen the development of many models to predict lithiation stresses generated during operation, and their effects on performance. Due to computational considerations most of the particles studied have idealized geometry with smooth surfaces, such as spheres. In reality, storage particles used in battery electrodes are acicular and have sharp edges and corners. In order to study the effect of these edges and corners on the generation of lithiation stress, we perform a parameter study on the development of lithiation strain and the resulting stress in cubic-shaped particles. We use a previously developed coupled stress-diffusion model, as well as three non-dimensional parameters, to quantify the stress response of cubic-shaped particles as a function of their material properties. Our results show that a change in material properties can lead to differences in both the value of maximum stress as well as its location in the particle. Both lithium insertion into and extraction from the particle are considered.

Understanding of the complex electrochemical, transport, and phase-change phenomena in Li-S cells requires experimental characterization in tandem with mechanistic modeling. However, existing Li-S models currently contradict some key features of experimental findings, particularly the evolution of cell resistance during discharge. We demonstrate that, by introducing a concentration-dependent electrolyte conductivity, the correct trends in voltage drop due to electrolyte resistance and activation overpotentials are retrieved. In addition, we reveal the existence of an often overlooked potential drop mechanism in the low voltage-plateau which originates from the limited rate of Li2S precipitation.