Realizing high‐energy density for practical lithium–sulfur batteries

Lithium–sulfur (Li–S) batteries has emerged as a promising post‐lithium‐ion battery technology due to their high potential energy density and low raw material cost. Recent years have witnessed substantial progress in research on Li–S batteries, yet no high‐energy Li–S battery products have reached the market at scale. Achieving high‐energy Li–S batteries necessitates a multidisciplinary approach involving advanced electrode material design, electrochemistry, and electrode and cell engineering. In this perspective, we offer a holistic view of pathways for realizing high‐energy Li–S batteries under practical conditions. Starting with a market outlook for high‐energy batteries, we present a comprehensive quantitative analysis of the critical parameters that dictate the cell‐level energy density for a Li–S battery. Thereby we establish a protocol to expedite the integration of lab‐scale Li–S research results into practical cell. Furthermore, we underscore several key considerations for promotion of commercial viability of high‐energy Li–S batteries from the perspective of battery industrialization.


| INTRODUCTION
The development of advanced battery technologies has been driven by the ever-increasing demand for more efficient energy storage.Lithium-ion (Li-ion) batteries have profoundly shaped the information-rich and mobile world since their commercialization in the 1990s. [1,2]They have dominated the market for portable electronics over the past 30 years because of their high-energy density and long lifespan compared to their counterparts including lead-acid batteries in the 1850s, nickel-cadmium batteries in the 1890s, and nickel-metal hydride batteries in 1960s.In recent years, the Li-ion battery manufacturing market has seen significant growth, with a production capacity of 75 GWh year -1 in 2015 and a drastic increase to 230 GWh year -1 in 2020. [3]espite the increasing market share, state-of-the-art Liion batteries are approaching their theoretical limit for energy density and are now challenged by the demanding requirements of the emerging electric vehicles (EVs) and grid-energy storage sectors. [4,5]Electrification of transportation has become a global trend.Many countries, including the United Kingdom, Norway, and the Netherlands, have announced that, the sales of fuel-based vehicles will be gradually prohibited by the middle of the 21st century.Automobile electrification is also indispensable for the development of autonomous vehicle technology. [6]Moreover, the decreasing availability of nonrenewable fossil fuels has triggered the development of sustainable and renewable resources such as wind and solar energy.The intermittent nature of these resources necessitates more powerful and economic battery technologies to store and integrate the renewable energy into electric grids. [7]Meanwhile, the worldwide battery market has been growing continuously, with an average annual growth rate of 7% in production capacity and 8% in market value from 2010 to 2020 (Figure 1). [3]Given the rapid development of EVs and stationary energy storage, the production capacity is forecasted to drastically increase to 1800 GWh year -1 along with a predicted market value of US$190 billion in 2030 (Figure 1). [3]Propelled by the evergrowing market size and the increasing energy requirement, exploring post-Li-ion battery technologies with high-energy density has become a necessity.
10] Apparently, Li-S batteries hold great promise to provide a high-energy density.The natural abundance and environmental benignity of sulfur further endow Li-S batteries with low-cost and green features, making them highly appealing for future high-energy applications.
][17][18][19][20][21] Most research studies primarily focus on the development of advanced electrode materials, aiming to improve the electrochemical activity of sulfur.However, these are predominantly implemented on lab-scale model systems, and no high-energy Li-S battery products have reached the market at scale.It is noteworthy that achieving high-energy Li-S batteries necessitates a multidisciplinary approach involving advanced electrode material design, electrochemistry, and electrode and cell engineering.It is The development of the worldwide battery market from 2010 to 2020 with a market outlook from 2025 to 2030: (A) the production capacity, (B) the market value at pack level.The pack include the cell, cell assembly, battery management system, and connectors.The power electronics (DC-DC convertors, inventors, etc.) are not included.The anticipated production capacities and market values from 2025 to 2030 are shown in lighted shaded columns.All the data are obtained from AVICENNE ENERGY. [3]10] Future Li-ion batteries refer to those using advanced high-capacity anode materials such as silicon and lithium metal instead of graphite.not yet clear that, whether the alleged superior performance obtained through developing new materials and concepts can be translated into a practical Li-S cell, how Li-S batteries can compete with the mainstream Li-ion batteries, and to what extent the Li-S battery technology is ready for practical highenergy applications.
In our previous review, we emphasized the importance of a high sulfur fraction in cathode materials, high areal sulfur loading in the sulfur cathode, and minimal electrolyte amount in enhancing the energy density of Li-S batteries. [22]uilding on this, we hereby aim to offer a comprehensive quantitative analysis of the key parameters that dictate the cell-level energy density of a Li-S battery and identify the critical conditions for an energy density higher than 350 Wh kg -1 .To bridge the gap between academic and industrial metrics, we establish a protocol to expedite the integration of lab-scale Li-S research results into realistic cell designs.From the perspective of industrialization, we further underscore several key considerations essential for promoting the commercial viability of Li-S batteries.

| CRITICAL PARAMETERS FOR A HIGH-ENERGY LI-S CELL
Most lab-scale experiments on Li-S batteries are carried out in the form of coin cells, making it difficult to estimate the practical energy density. [23,24]Here, we calculate the celllevel energy density of a Li-S battery based on the configuration of a multilayered pouch cell with doublesided-coated cathode/anode, which is a commercial battery format that allows maximum use of space and minimum use of inactive components like current collectors and cell packaging. [25]We will present a comprehensive analysis of the energy density-determining parameters in a Li-S pouch cell and elucidate how the gap forms between coin cellbased achievements and the requirements for high-energy cells under realistic conditions.
The gravimetric energy density (E sp , in Wh kg -1 ) of a multilayered Li-S pouch cell can be defined as: where C S is the specific capacity of electroactive sulfur (in mAh g -1 ), U is the nominal cell voltage (~2.15 V), m S is the weight of electroactive sulfur, and m cell is the weight of the whole cell, including the weight of the cathode (m cathode ), anode (m anode ), electrolyte (m Ey ), current collectors (m cc ), separator (m sep ), and cell housing materials (m housing ) including tabs and packaging, described as: Assuming that the number of double-side coated cathode/anode is N , the area of the pouch cell is A, and the weights of the cell components can be described as: where is L S the areal sulfur loading each side (in mg cm -2 ), ω S is the sulfur content in the cathode, r N/P is the negative to positive capacity ratio (N/P ratio), C s,theo and C Li,theo are the theoretical capacities for sulfur (1675 mAh g -1 ) and Li (3860 mAh g -1 ), respectively, r E/S is the electrolyte to sulfur ratio (E/S ratio, in µL mg -1 ), ρ Ey is the density of the electrolyte (~1.2 g cm -3 ), L Al , L Cu , L sep , and L housing are the areal densities for the cathode current collector (15 µm Al foil, 4.1 mg cm -2 ), the anode current collector (8 µm Cu foil, 7.2 mg cm -2 ), the separator (20 µm, 0.8 mg cm -2 ), and the housing materials (~70 mg cm -2 ). [25] S can be correlated with the sulfur utilization rate (η S ), described as: Accordingly, Equation ( 1) can be converted into: Therefore, there are six parameters, that is, sulfur utilization rate, sulfur content, areal sulfur loading, N/P ratio, E/S ratio, and the number of the electrode layers that can exert an influence on the energy density of a Li-S pouch cell.The dependences of the energy density on the six parameters are detailed in Figure 2, based on a reference cell with sulfur utilization rate of 75%, sulfur content of 80%, areal sulfur loading of 6 mg cm -2 , N/P ratio of 2, E/S ratio of 3 µL mg -1 , and the number of the electrode layers of 8.Each figure in Figure 2 depicts the effect of two parameters on energy density, while keeping the remaining four parameters constant at the corresponding values from the reference cell.
Sulfur utilization, that is, the specific capacity normalized to the sulfur mass, has been the focus of most research studies in Li-S literature.Basically, energy density increases linearly with sulfur utilization.As shown in Figure 2A, a sulfur utilization above 0.6 (specific capacity 1005 mAh g -1 ) is a requisite for an energy density above 350 Wh kg -1 .Nevertheless, the positive effect of high sulfur utilization can be greatly compromised by low sulfur loadings below 4 mg cm -2 .
Sulfur loading has been identified as a critical parameter for increasing energy density in recent literature.The energy density increases sharply as sulfur loading increases, until reaching a saturation point, beyond which the energy density does not increase much further (Figure 2A,C).This means that the cell-level energy density is unlikely to increase infinitely by packing more sulfur into the cell, as the weight of additional sulfur and the concomitant anode and electrolyte (Equations 5 and 6) contribute to the weight of the whole cell.In addition, high sulfur loading comes with challenges, including decreased sulfur utilization and output voltage that can impair the energy density due to increased internal resistance.Accordingly, a desirable sulfur loading should be 6-9 mg cm -2 to achieve a high-energy density.
Sulfur content is widely considered to be another important parameter.The motivation of increasing sulfur content is to minimize the weight of inactive components such as conductive additives and binders in the cathode so that the energy density can be increased.However, the impact of sulfur content on energy density seems to have been overstated, as elaborated in Figure 2B.This is because the inactive component in the cathode only takes a minor fraction in the weight of the whole cell.Moreover, the benefit of high sulfur content (>90%) to high-energy density (>350 Wh kg -1 ) can be completely offset by sulfur loading below 5 mg cm -2 (Figure 2B,C).
Projected energy density of a multilayered lithium-sulfur pouch cell under different conditions: (A) at various sulfur loadings and sulfur utilizations with fixed sulfur content of 80%, E/S ratio of 3 µL mg -1 , N/P ratio of 2, and number of cathode layers of 8, (B, C) at various sulfur contents and sulfur loadings with fixed sulfur utilization of 75%, E/S ratio of 3 µL mg -1 , N/P ratio of 2, and number of cathode layers of 8, (D) at various E/S ratios and sulfur loadings with fixed sulfur utilization of 75%, sulfur content of 80%, N/P ratio of 2, and number of cathode layers of 8, (E) at various N/P ratios and sulfur loadings with fixed sulfur utilization of 75%, sulfur content of 80%, E/S ratio of 3 µL mg -1 , and number of cathode layers of 8, (F) at various numbers of cathode layers and sulfur loadings with fixed sulfur utilization of 75%, sulfur content of 80%, sulfur loading of 6 mg cm -2 , E/S ratio of 3 µL mg -1 , and N/P ratio of 2. The light-gray shaded regions represent the conditions that allow an energy density higher than 350 Wh kg -1 .
The above three parameters have been more or less recognized and emphasized in recent literature, while the importance of E/S ratio and N/P ratio are often overlooked, despite their significant impact on cell-level energy density.As shown in Figure 2D,E, the energy density decreases drastically with increasing E/S ratio and N/P ratio, even at high sulfur loadings.At E/S ratios above 4 µL mg -1 and/or N/P ratios above 4, an energy density above 350 Wh kg -1 becomes unreachable.However, lab-scale Li-S coin cells are mostly investigated with huge excesses of electrolyte and lithium to achieve a high specific capacity and long cycling lifespan, leading to false expectations that are not achievable under realistic conditions.
The number of cathode layers is another parameter that can affect the energy density of a multilayered Li-S pouch cell but is not applicable in coin cells.As shown in Figure 2F, an energy density above 350 Wh kg -1 requires at least four layers of double-side coated cathode in the Li-S pouch cell.However, there is also a saturation point like that in Figure 2A, where a further increase in the number of cathode layers has a minor effect on the energy density.
Notably, here we assume that the values of L Al , L Cu , L sep , and L housing are constants for better clarity.As Al and Cu current collectors, separators, and housing materials are all inactive components, decreasing the values of these four parameters is beneficial for increasing energy density.It is also worth mentioning that the interactive effects among these parameters have been simplified in the above discussions to present a clear quantitative analysis.Typically, increasing sulfur content and sulfur loading results in decreased specific capacity, [26] reducing E/S ratio causes sluggish charge transfer kinetics and decreased sulfur utilization, [27] and reducing N/P ratio leads to  | 765 fast depletion of electroactive Li and decreased specific capacity. [28,29]As illustrated Equation (1), cell voltage is a also a determining factor for energy density, which can be influenced by the cell parameters discussed in the manuscript.For example, increase in sulfur content and sulfur loading and decrease in E/S ratio often result in decreased cell voltage owing to restricted cathode reaction kinetics.The behaviors and properties of practical pouch cells are definitely more complex, and these interactive effects need to be taken into consideration in realistic cell designs.
Based on the above discussions, we exemplify a 360 Wh kg -1 prototype Li-S pouch cell, with specifications of the abovediscussed six parameters listed in the left panel of Figure 3A and the corresponding mass distributions shown in the right panel.The electrolyte, sulfur, and take the largest mass fractions in the prototype Li-S cell, which is the reason why the effects of E/S ratio, sulfur loading, and N/P ratio on energy density are the most significant (Figure 2A,D,E).The cathode additives only account for a small mass fraction, and this also explains the limited positive effect of increasing sulfur content on energy density (Figure 2B,C).Figure 3B gives a sensitivity analysis of how fine adjustments of a single parameter affect the energy density of the 360 Wh kg -1 prototype cell.Apparently, energy density is highly sensitive to E/S ratio and N/P ratio, corresponding to Figure 2D,E.While the sensitivity of energy density to sulfur loading gradually decreases from 5 to 7 mg cm -2 as the saturation point is approaching (Figure 2A).Meanwhile, the effect of sulfur content and number of cathode layers becomes minor under the optimized conditions of the prototype Li-S cell.

| BRIDGING THE GAP BETWEEN ACADEMIC AND INDUSTRIAL METRICS
As most coin cells vary widely in sulfur loading and electrolyte and lithium excess amounts, vastly different electrochemical results can be obtained even with the same electrode material.The alleged superior performances obtained under unrestricted testing conditions can hardly be reproduced in practical pouch cells. [30]To efficiently incorporate the new materials and concepts into a practical Li-S cell, it is essential to re-evaluate them under experimental conditions relevant to realistic high-energy cells.To bridge the gap between the academic and industrial metrics, we herein propose a coin cell-testing protocol to expedite the integration of lab-scale Li-S research results into realistic high-energy cell designs, which include four levels, as illustrated in Figure 4.
At the fundamental level, the target is mostly to study fundamental mechanisms and/or implement analytical/methodology research.It can be a screening stage to pre-evaluate the feasibility of new materials and concepts and their compatibility with other cell components.There are no particular requirements on the cell parameters in this stage, as achieving high-energy density is not the focus at this stage.
When the new material or concept has been proven feasible, the next material level aims to evaluate the electron and ion-transport properties of the electrode material to ensure its capability to achieve high sulfur utilization.At this stage, sulfur utilization needs to reach 70% at a high sulfur content of 70%, with no more F I G U R E 4 A coin cell-testing protocol for integration of lab-scale lithium-sulfur research results into realistic cell designs.
requirements on the sulfur loading and anode and electrolyte excess.
When high sulfur utilization is achievable with a high sulfur content, the following electrode level requires further restrictions on the sulfur loading (≥5 mg cm -2 ) and N/P ratio (≤3).This stage is more relevant to the electrode architecture. [31,32]The challenge here is to minimize the effect of increased sulfur loading on sulfur utilization.Reported strategies include using interconnected current collectors and developing novel electrode preparation methods.
When uncompromised sulfur utilization can be achieved with high sulfur loading and limited lithium supply, the last cell level further requires a low E/S ratio (≤4 µL mg -1 ).High sulfur loading with a lean electrolyte supply can be even more challenging to achieve a high sulfur utilization. [33,34]This requires high electrolyte wettability to guarantee sufficient ionic conductive pathways and high electrolyte stability to minimize irreversible electrolyte consumption on the lithium anode.At this stage, the energy density of the Li-S cell can be estimated based on Equation (11).When the estimated energy density is desirable, a multilayered Li-S pouch cell can be assembled for re-evaluation and translation of the new design concepts into realistic battery systems.

| INDUSTRIALIZATION OF LI-S BATTERIES
Industrialization of a battery technology requires comprehensive consideration of research and development, production, marketing, and sustainability.Figure 5 illustrates Li-S battery value chain, which includes six main segments: raw material production/refining, cell component production, cell manufacturing, battery pack manufacturing, integration and application, and reuse and recycling.Given the continuous progress in developing Li-S cell components, industrial-scale manufacturing of Li-S cells and battery packs will become an increasingly important aspect to speed industry adoption.
Here we discuss the key considerations relevant to battery manufacturing and diagnostic techniques for battery packs.
The manufacturing process of Li-ion batteries is highly optimized and standardized, including cathode production, anode production, cell assembly and conditioning.The compatibility of Li-S battery production with current Li-ion battery production infrastructures plays a significant role in the pace of their practical applications.
The electrode production processes for Li-ion batteries typically include the following procedures: mixing, coating, calendaring, slitting, and drying.The sulfur cathode production involves basically the same steps as Li-ion battery electrodes, yet the Li metal anode brings additional challenges in anode production.As metallic Li is highly sensitive to the moisture, oxygen, nitrogen, and carbon dioxide from ambient air, it must be processed in a dry room or under an inert atmosphere such as argon.Besides, the production of Li metal foil with controllable thickness also requires new manufacturing competencies. [35]The ductile Li metal first needs to be extruded to form a foil shape, followed by high-intensity calendaring to further reduce the foil thickness.Then the surface of the Li foil needs to be passivated for further consecutive manufacturing steps in the dry room.After passivation, the Li foil needs to be slitted by a laser into single electrodes, as the mechanical roll-knife slitting method established for Li-ion batteries is not suitable for the adhesive Li foil.The complicated production process of Li anode and requirements for dry rooms and new machinery significantly increase the anode processing cost, which may compromise the cost effectiveness of Li-S batteries.
The cell production processes typically include electrode cutting, stacking and assembly, welding, enclosing and electrolyte filing.For the stacking step, production techniques need to be modified for Li-S batteries to allow fast handling of ultrathin, sticky, and adhesive Li foils.Besides, laser welding is needed for the following welding step, as the mechanical vibrational energy from the conventional ultrasonic welding would F I G U R E 5 Illustration of lithium-sulfur battery value chain.
damage the thin foil.These further contribute to increased processing cost.Nevertheless, as the Li-S technology gradually matures, these additional costs can be offset by economies of scale. [36]or battery pack manufacturing, the most important part is the integration of a proper battery management system (BMS).BMS is designed for safe and optimal use of the battery pack, and one of its main responsibilities is to monitor the state of the battery as represented by various items such as voltage, current, temperatures, and resistance. [13]Developing effective diagnostic techniques for accurate estimation of battery states, including state-of-charge (SoC) and state-of-health is vital for safe operation and optimal usage of Li-S batteries. [37,38]owever, existing techniques commonly used for Li-ion battery chemistries such as open-circuit voltage (OCV) monitoring and Coulomb counting (current monitoring) have issues when implemented in Li-S batteries.This can be attributed to the multistep sulfur conversion reactions, the complex failure modes, and the particular two-plateau shape of OCV curve of Li-S batteries.Solutions include combining diverse diagnostic techniques, applying control theory, and developing effective modeling for Li-S cells.
The practical utility of Li-S batteries is contingent upon their alignment with specific application requirements.The primary advantage of Li-S batteries rests in their remarkable gravimetric energy density, yet their volumetric energy density lags behind that of Li-ion batteries due to the lower density of active materials they employ.Presently, Li-S batteries exhibit inferior cycle life and power density when compared to Li-ion batteries.Nonetheless, it is anticipated that ongoing research and development efforts will foster improvements in these areas.In the near term, the deployment of Li-S batteries is anticipated in specialized markets such as drones and satellites.As energy density and power density continue to advance, the potential applications of Li-S batteries could extend to the realm of transportation, encompassing sectors like electric buses.As safety and costeffectiveness of Li-S batteries are further enhanced, opportunities could open up for their integration into stationary energy storage applications, given the crucial significance of cost control and safety in large-scale energy storage.

| CONCLUSION
Li-S batteries are presented as a promising technology with advantages of high energy and low cost over other battery technologies.Remarkable advances in Li-S battery research have brought the commercial viability of this battery technology into spotlight.Achieving high-energy density under realistic conditions is a crucial prerequisite for the practical use of Li-S batteries.Through comprehensive quantitative analysis, we determine the underlying correlation between the key parameters and cell-level energy density and identify the critical requirements for designing a high-energy Li-S battery.To bridge the gap between academic and industrial metrics, we establish a protocol to expedite the integration of labscale Li-S research results into realistic cell designs featuring high sulfur loading, limited anode excess and lean electrolyte supply.From the perspective of battery industrialization, we underscore the importance of establishing new manufacturing competencies for efficient production and developing proper BMS for safe operation and optimal usage.We hope this perspective would encourage more dedicated efforts to design and construct high-energy Li-S batteries under realistic conditions and promote their rapid implementation for practical use.
Specifications of a 360 Wh kg -1 prototype lithium-sulfur pouch cell and the corresponding mass distributions.Cathode additives include conductive additives and binders.(B) Sensitivity analysis showing how fine adjustments of a single parameter affect the energy density of the prototype (reference) cell in (A).When adjusting one parameter, all other parameters are fixed at the corresponding values of the reference cell.