Lithium Sulfur (Li-S) batteries are one of the most promising next generation battery chemistries with potential to achieve 500-600 Wh/kg in the next few years. Yet understanding the underlying mechanisms of operation remains a major obstacle to their continued improvement. From a review of a range of analytical studies and physical models, it is clear that experimental understanding is well ahead of state-of-the-art models. Yet this understanding is still hindered by the limitations of available techniques and the implications of experiment and cell design on the mechanism. The mechanisms at the cor e of physical models for Li-S cells are overly simplistic compared to the latest thinking based upon experimental results, but creating more complicated models will be difficult, due to the lack of and inability to easily measure the necessary parameters. Despite this, there are significant opportunities to improve models with the latest experimentally derived mechanisms. Such models can inform materials research and lead to improved high fidelity models for controls and application engineers.

A picture of the challenges faced by the lithium-sulfur technology and the activities pursued by the research community to solve them is synthesized based on 1992 scientific articles. It is shown that, against its own advice of adopting a balanced approach to development, the community has instead focused work on the cathode. To help direct future work, key areas of neglected research are highlighted, including cell operation studies, modelling, anode, electrolyte and production methods, as well as development goals for real world target applications such as high altitude unmanned aerial vehicles. Lithium Sulfur (Li-S) batteries are one of the most promising next generation battery technologies 1 due to their high theoretical energy density, low materials cost, and relative safety.2 Li-S has the potential to achieve significantly higher gravimetric energy density than intercalation based lithium ion technologies, 3 with some companies already reporting 400 Wh/kg cells. 4,5 However, Li-S has a lower comparable volumetric energy, 6 suggesting that applications where minimising mass is more important than volume will adopt it faster. Li-S technology is close to industrial production, 7 with a number of companies scaling up manufacturing capabilities for large capacity cells. 4,5 Meanwhile, the number of Li-S research papers published per year has increased dramatically from less than 50 in 2010 to over 900 in 2016. We have reviewed almost two thousand articles to identify the major gaps in research and discussed how targeting them could speed up the development and adoption of Li-S technology. We also discuss how from an industry/applied research viewpoint focussing on a performance metric, such as power density, would speed up development iterations, getting products to market sooner and help unlock further research funding. Current StatusLi-S cells are already commercially viable in niche applications. In order to expand their market potential, however, there are still many challenges to overcome, such as limited cycle life, high self-discharge rates and over-heating at end of charge. Many of these are thought to be caused by the shuttle, where cathode species diffuse to the anode and react directly with the metallic lithium.8 Multiple solutions have therefore been proposed to prevent shuttle, such as physically 9,10 or chemically 11-13 encapsulating the sulfur, designing tailored carbon structures, 14 using electrolyte additives, 15,16 separators, 17 protective layers, 18 or solid electrolytes to physically protect the anode. 19-21However, many of these solutions affect energy or power density adversely or do not function in practical commercial cells. Li-S batteries also undergo significant volume changes during operation, which poses a particular challenge for battery pack system designers and is being studied only since recently. 22 These observations have only been possible since large form factor pouch cells are available. The effect of precipitation on useable capacity and reversible capacity loss, 23 and ...

Lithium-sulfur cells present an attractive alternative to Li-ion batteries due to their large energy density, safety, and possible low cost. Their successful commercialisation is dependent on improving their performance, but also on acquiring sufficient understanding of the underlying mechanisms to allow for the development of predictive models for operational cells. To address the latter, we present a zero dimensional model that predicts many of the features observed in the behaviour of a lithium-sulfur cell during charge and discharge. The model accounts for two electrochemical reactions via the Nernst formulation, power limitations through Butler-Volmer kinetics, and precipitation/dissolution of one species, including nucleation. It is shown that the flat shape of the low voltage plateau typical of the lithium-sulfur cell discharge is caused by precipitation. During charge, it is predicted that the dissolution can act as a bottleneck, because for large enough currents the amount that dissolves becomes limited. This results in reduced charge capacity and an earlier onset of the high plateau reaction, such that the two voltage plateaus merge. By including these effects, the model improves on the existing zero dimensional models, while requiring considerably fewer input parameters and computational resources than one dimensional models. The model also predicts that, due to precipitation, the customary way of experimentally obtaining the open circuit voltage from a low rate discharge might not be suitable for lithium-sulfur. This model can provide the basis for mechanistic studies, identification of dominant effects in a real cell, predictions of operational behaviour under realistic loads, and control algorithms for applications.

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.

Lithium-sulfur (Li-S) battery could bring a step-change in battery technology with a potential specific energy density of 500 -600 Wh/kg. A key challenge for further improving the specific energy-density of Li-S cells is to understand the mechanisms behind reduced sulfur utilisation at low electrolyte loadings and high discharge currents. While several Li-S models have been developed to explore the discharge mechanisms of Li-S cells, they so far fail to capture the discharge profiles at high currents. In this study, we propose that the slow ionic transport in concentrated electrolyte is limiting the rate capability of Li-S cells. This transport-limitation mechanism is demonstrated through a one-dimensional Li-S model which qualitatively captures the discharge capacities of a sulfolane-based Li-S cell at different currents. Furthermore, our model predicts that a discharged Li-S cell is able regain some capacity with a short period of relaxation. This capacity recovery phenomenon is validated experimentally for different discharge currents and relaxation durations. The transport-limited discharge behavior of Li-S cells highlights the importance of optimizing the electrolyte loading and electrolyte transport property in Li-S cells.

Li-S batteries exhibit poor rate capability under lean electrolyte conditions required for achieving high practical energy densities. In this contribution, we argue that the rate capability of commercially-viable Li-S batteries is mainly limited by mass transfer rather than charge transfer during discharge. We first present experimental evidence showing that the charge-transfer resistance of Li-S batteries and hence the cathode surface covered by Li 2 S are proportional to the state-of-charge (SoC) and not to the current, directly contradicting previous theories. We further demonstrate that the observed Li-S behaviors for different discharge rates are qualitatively captured by a zero-dimensional Li-S model with transport-limited reaction currents. This is the first Li-S model to also reproduce the characteristic overshoot in voltage at the beginning of charge, suggesting its cause is the increase in charge transfer resistance brought by Li 2 S precipitation. Achieving a high practical energy density in Li-S batteries requires increased sulfur loading and reduced electrolyte loading at cell level. However, increasing the sulfur-to-electrolyte mass ratio lowers the sulfur utilization as well as the rate capability.1-3 The charge rate of Li-S batteries is mainly limited by the large initial overpotential associated with the activation of precipitated Li 2 S, 4 as well as the slow subsequent dissolution that could lead to incomplete Li 2 S conversion at the end of charge. 5 The mechanisms behind the low discharge rate capability have been explained in terms of mass transport limitation and surface passivation caused by the accumulation of precipitated Li 2 S.The transport limitation could arise from high electrolyte viscosity and pore-blocking due to localized Li 2 S precipitation. We recently demonstrated that a Li-S cell discharged at high current can provide extra capacity after an hour of relaxation.6 With a one-dimensional Li-S model, it was found that slow ionic transport could force polysulfides to accumulate temporarily in the separator, leading to the known reduced capacity at high discharge rates. The capacity recovery phenomenon is the result of redistribution of polysulfides across the cell during relaxation. Danner et al. 7 further demonstrated with a multiscale Li-S model that localized precipitation tends to occur at the carbon-sulfur particle surface, leading to pore-blocking and transport-limited discharge capacity for high sulfur loading.The insulating and insoluble discharge product, Li 2 S, has been shown experimentally to cover active cathode surfaces during discharge and increase the charge-transfer resistance. The relation between discharge rate and morphology of the precipitate, however, remains under debate. Based on ex-situ SEM imaging of a carbon fiber ultramicroelectrode of a Li-S cell, Fan et al. 8 revealed that Li 2 S formed as a thin, continuous coating after a fast discharge but appeared as a small number of large particles after a slow discharge. It was therefore hypothesized that ...

Lithium-sulfur batteries could deliver significantly higher gravimetric energy density and lower cost than Li-ion batteries. Their mass adoption, however, depends on many factors, not least on attaining a predictive understanding of the mechanisms that determine their performance under realistic operational conditions, such as partial charge/discharge cycles. This work addresses a lack of such understanding by studying experimentally and theoretically the response to partial cycling. A lithium-sulfur model is used to analyze the mechanisms dictating the experimentally observed response to partial cycling. The zero-dimensional electrochemical model tracks the time evolution of sulfur species, accounting for two electrochemical reactions, one precipitation/dissolution reaction with nucleation, and shuttle, allowing direct access to the true cell state of charge. The experimentally observed voltage drift is predicted by the model as a result of the interplay between shuttle and the dissolution bottleneck. Other features are shown to be caused by capacity fade. We propose a model of irreversible sulfur loss associated with shuttle, such as caused by reactions on the anode. We find a reversible and an irreversible contribution to the observed capacity fade, and verify experimentally that the reversible component, caused by the dissolution bottleneck, can be recovered through slow charging. This model can be the basis for cycling parameters optimization, or for identifying degradation mechanisms relevant in applications. Lithium sulfur (LiS) batteries have the potential to provide a step change in performance, compared to Li-ion batteries, with an expected practical energy density of 700 Wh kg −1 compared to that of the intercalation Li-ion batteries, of 210 Wh kg −1 . 1,2 Added benefits, such as a potential low cost due to the abundance of the active materials, low toxicity and relative safety, 3 make them an attractive energy storage solution for a wide variety of applications, such as space exploration 4 and low temperature energy delivery. 5 However, the relatively low power capabilities, significant self discharge and low cycle life have so far hindered mainstream adoption of LiS cells. Degradation mechanisms such as lithium anode corrosion, self discharge and low coulombic efficiency have all been related to the polysulfide shuttle. As a result, most effort in the research community is currently directed toward decreasing the amount of shuttle through material design, and assessing the properties of the proposed materials through coin cell characterization.We argue that equally important for accelerating the adoption of this battery chemistry is the understanding of how real cells behave under real operating conditions, which often include incomplete charge/discharge cycles, noisy current loads and rest periods at various states of charge (SoC). Understanding and detecting the mechanisms leading to degradation, such as capacity fade, are intermediate steps crucial to predicting cycle life. Such understanding can...

Cooling electrical tabs of the cell instead of the lithium ion cell surfaces has shown to provide better thermal uniformity within the cell, but its ability to remove heat is limited by the heat transfer bottleneck between tab and electrode stack. A two-dimensional electro-thermal model was validated with custom made cells with different tab sizes and position and used to study how heat transfer for tab cooling could be increased. We show for the first time that the heat transfer bottleneck can be opened up with a single modification, increasing the thickness of the tabs, without affecting the electrode stack. A virtual large-capacity automotive cell (based upon the LG Chem E63 cell) was modelled to demonstrate that optimised tab cooling can be as effective in removing heat as surface cooling, while maintaining the benefit of better thermal, current and state-of-charge homogeneity. These findings will enable cell manufacturers to optimise cell design to allow wider introduction of tab cooling. This would enable the benefits of tab cooling, including higher useable capacity, higher power, and a longer lifetime to be possible in a wider range of applications.