Batteries that extend performance beyond the intrinsic limits of Li-ion batteries are among the most important developments required to continue the revolution promised by electrochemical devices. Of these next-generation batteries, lithium sulfur (Li–S) chemistry is among the most commercially mature, with cells offering a substantial increase in gravimetric energy density, reduced costs and improved safety prospects. However, there remain outstanding issues to advance the commercial prospects of the technology and benefit from the economies of scale felt by Li-ion cells, including improving both the rate performance and longevity of cells. To address these challenges, the Faraday Institution, the UK’s independent institute for electrochemical energy storage science and technology, launched the Lithium Sulfur Technology Accelerator (LiSTAR) programme in October 2019. This Roadmap, authored by researchers and partners of the LiSTAR programme, is intended to highlight the outstanding issues that must be addressed and provide an insight into the pathways towards solving them adopted by the LiSTAR consortium. In compiling this Roadmap we hope to aid the development of the wider Li–S research community, providing a guide for academia, industry, government and funding agencies in this important and rapidly developing research space.

Water dynamics in the membrane electrode assembly (MEA) and flow channels of polymer electrolyte fuel cells (PEFCs) is governed by the complex interplay of many physical and operational factors. The chemical nature and structure of the gas diffusion layer (GDL) plays a large part in this and is affected by the extent to which is mechanically compressed. Here, X-ray computed tomography shows the effect of cell compression on the MEA, and how it differs under the land and channel regions. Multi-orientation neutron radiography reveals the effect of compression on the way in which water accumulates and is transported between land and channel and between cathode and anode. By performing neutron imaging in both the inplane and through-plane directions it is possible to determine what constitutes a given 'thickness' of water mapped across the extent of an MEA. Changing MEA compression from 25% to 35% has a significant effect on water distribution and dynamics in operational cells. The effect of compression on performance is most marked in the mass transport region and there are consequences for liquid accumulation in channels and back-diffusion of water from the cathode to the anode.

The performance of the of polymer electrolyte membrane (PEM) fuel cell is governed by a complex interaction of the structure of the membrane electrode assembly (MEA), cell compression, and operating parameters. Adequate cell compression for improved current collection and gas sealing, can structurally deform MEA with adverse consequences. Nonuniform MEA compression exerted by the flow-field design and arrangement induces heterogeneous transport properties. Hence, understanding morphological evolution and effective transport properties as an effect of MEA compression is an important factor for improving fuel cell performance and durability. In this paper, an X-ray computed tomography study of the entire MEA compression is presented, comprising of gas diffusion and microporous layers, catalyst layers, and the electrolyte membrane, subjected to non-uniform compression under two distinct flow-field arrangements. This study presents a comprehensive dataset of the heterogeneous effective properties required for robust computational modelling; including porosity, permeability, tortuosity, and diffusivity, along with the extent of blocking of the flow channel due to cell compression and effect of compression on the structural properties of the membrane.

Metal foam flow-fields have shown great potential in improving the uniformity of reactant distribution in polymer electrolyte fuel cells (PEFCs) by eliminating the 'land/channel' geometry of conventional designs. However, a detailed understanding of the water management in operational metal foam flow-field based PEFCs is limited. This study aims to provide the first clear evidence of how and where water is generated, accumulated and removed in the metal foam flow-field based PEFCs using in-operando neutron radiography, and correlate the water 'maps' with electrochemical performance and durability. Results show that the metal foam flow-field based PEFC has greater tolerance to dehydration at 1000 mA cm -2 , exhibiting a ~50% increase in voltage, ∼127% increase in total water mass and ~38% decrease in high frequency resistance (HFR) than serpentine flow-field design. Additionally, the metal foam flow-field promotes more uniform water distribution where the standard deviation of the liquid water thickness distribution across the entire cell active area is almost half that of the serpentine. These superior characteristics of metal foam flow-field result in greater than twice the maximum power density over serpentine flow-field. Optimizing fuel cell operating condition and foam microstructure would partly mitigate flooding in the metal foam flow-field based PEFC, which will form the basis of future work.

Flow-fields are key factors in determining the operation of fuel cells. While extensive work has been conducted to develop and optimise the reactant flow and current collection performance of polymer electrolyte membrane fuel cell (PEMFC) components, there is a factor that remains largely unaccounted for. Depending on how a membrane electrode assembly (MEA) is incorporated into a cell, there will often be a small gap between the edge of the gas diffusion layer (GDL) and the seal or bipolar plate. This gap acts as a 'secondary flow-field' (SFF) that can bypass or affect/augment the conventional or 'primary flow-field'. Understanding how this affects performance (either positively or adversely) is essential for holistic flow-field design. This paper describes the issues associated with the SFF, examines how cell compression affects its width due to lateral expansion of the GDL and discusses the results of a 3-D computational model that investigates the effect of the SFF during dead-ended anode (DEA) operation for a fuel cell without a macroscopic (conventional) anode flowfield.

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Lithium Sulfur batteries the represent the next generation battery chemistry that is closest to commercialisation. Having particular advantages in gravimetric energy density, they still suffer from low-rate performance and capacity fade, linked to the complex multi-step electrochemistry and heterogenous electrode structures. Physics-based models of batteries have long been used to understand and predict their behaviour; traditionally, 1D volume-averaged continuum approaches are used, not accounting for the heterogenous structure of the electrode. For the first time, we present a three-dimensional electrochemical model of a Li-S battery based on real electrode microstructure. The extent of heterogeneities present in the electrode architecture and the adequacy of using the representative elementary volume to capture the effect of complex electrode microstructure on the cell performance were analysed and compared with the 1D model. Finally, the future modelling framework that would aid in optimising the S/C structure for improved and uniform cell performance is discussed.