In order to improve the electrochemical performances of lithium-sulfur batteries, it is crucial to understand profoundly their working mechanism and the limitation factors. This communication presents synchrotron-based in situ XRD studies of structural modifications occurring inside the cell upon cycling, since the active material changes constantly its form between solid and liquid phases.
Li 2 S 2 species, sometimes considered as a solid product, [ 18,19 ] has never been experimentally detected by XRD, thus questioning its real existence as a solid. Sulfur reduction (complete [ 13,16 ] or incomplete [ 10,20 ] and recrystallization at the end of charge [ 11,12,16 ] was also an unresolved question. Until today, there have been few reports released on the study of Li/S cells, where in situ XRD was applied. [7][8][9]21 ] In the majority of these works, the formation of a solid sulfur at the end of charge was confi rmed. The appearance of crystalline Li 2 S was not evidenced by Nelson et al., [ 9 ] while other groups [ 8,21 ] reported on its formation at different states of discharge along the lower plateau. Both the reports offered only qualitative interpretation of the results. By applying in situ and operando XRD, information about the structural changes of sulfur positive electrode versus exchanged capacity upon cycling could be obtained. [7][8][9] This enables a deeper insight into the complex mechanism of the Li/S cell, which from the point of view of further improvements, is essential.Here we report, for the fi rst time, a quantitative evaluation of Li 2 S formation and consumption upon cycling and propose a mechanism of solid product(s) formation. Monitoring of further cycles, as well as C-rate infl uence (C/20 and C/8), is also taken into consideration. These results were obtained from a pouch cell using nonwoven carbon sheet as a current collector and relatively high sulfur loading, along with a controlled electrolyte amount. The effect of the electrolyte excess and electrode morphology on the solid phases formation was not the scope of this paper, therefore not investigated any further. Nevertheless, these important parameters may be studied in the future.The obtained electrochemistry displays an expected voltage profi le of a typical Li/S cell, where discharge and charge capacities of 980 and 976 mAh g −1 , respectively, were obtained. The corresponding XRD patterns evolution is shown on Figure S1a (Supporting Information). As the discharge proceeds, the peak intensities of the elemental orthorhombic α-sulfur (PDF-2; No. 00-008-0247) linearly decrease as a function of capacity ( Figure S2, Supporting Information). They vanish at the end of the fi rst discharge plateau, proving a complete reduction of sulfur. This linear evolution can be associated with a one-step electrochemical process, based on the reduction reaction of S 8 to S 8 2− . [ 22 ] The practical capacity of this fi rst region (so-called "solid/soluble"; 175 mAh g −1 ) is slightly lower than the theoretically expected one (209 mAh g −1 ), possibly because of a self-discharge associated with partial S 8 dissolution [ 23 ] (further details are provided in the Supporting Information). In the region Lithium/sulfur batteries are considered as a promising candidate for a next-generation energy storage system. This technology is based on the electrochemical reaction between elemental sulfur and metallic lithium (16Li + S 8
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.
During the operation of a Lithium-Sulfur (Li-S) cell, structural changes take place within both positive and negative electrodes. During discharge, the sulfur cathode expands as solid products (mainly Li2S or Li2S/Li2S2) are precipitated on its surface, whereas metallic Li anode contracts due to Li oxidation/stripping. The opposite processes occur during charge, where Li anode tends to expand due to lithium plating and solid precipitates from the cathode side are removed, causing its thickness to decrease. Most research literature describe these processes as they occur within single electrode cell constructions. Since a large format Li-S pouch cell is composed of multiple layers of electrodes stacked together, and antagonistic effects (i.e. expansion and shrinkage) occur simultaneously during both charge and discharge, it is important to investigate the volumetric changes of a complete cell. Herein, we report for the first time the thickness variation of a Li-S pouch cell prototype. In these studies we used a laser gauge for monitoring the cell thickness variation under operation. The effects of different voltage windows as well as discharge regimes are explored. It was found that the thickness evolution of a complete pouch cell is mostly governed by Li anodes volume changes, which mask the response of the sulfur cathodes. Interesting findings on cell swelling when cycled at slow currents and full voltage windows are presented. A correlation between capacity retention and cell thickness variation is demonstrated, which could be potentially incorporated into Battery Management System (BMS) design for Li-S batteries
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.
Lithium/sulphur batteries are promising candidates for future energy storage systems, mainly due to their high potential capacity. However low sulphur utilization and capacity fading hinder practical realizations. In order to improve understanding of the system, we investigate Li/S electrode morphology changes for different ageing steps, using X-ray phase contrast tomography. Thereby we find a strong decrease of sulphur loading after the first cycle, and a constant loading of about 15% of the initial loading afterwards. While cycling, the mean sulphur particle diameters decrease in a qualitatively similar fashion as the discharge capacity fades. The particles spread, migrate into the current collector and accumulate in the upper part again. Simultaneously sulphur particles lose contact area with the conducting network but regain it after ten cycles because their decreasing size results in higher surface areas. Since the capacity still decreases, this regain could be associated with effects such as surface area passivation and increasing charge transfer resistance.
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 ...
Electrochemical impedance spectroscopy (EIS) study was done in order to go deeper into the electrochemical processes of Li/S battery in a common CR2032 coin-type cell. In order to separate the contributions of each electrode (i. e. lithium negative electrode and sulfur composite positive electrode), impedance measurements on symmetrical cells consisting of two previously cycled electrodes was used. This methodology enables to propose an overall interpretation of the different electrochemical phenomena present during cycling. Low temperature tests were also applied, which brought fruitful information concerning the kinetics of the reactions and allowed to confirm the chemical-physical interpretations performed on the cell cycled at 25°C.
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