Li/S Battery Design: Comparison
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Please note this is a comparison between Version 2 by Dean Liu and Version 1 by Yoon Hwa.

Lithium/sulfur (Li/S) cells that offer an ultrahigh theoretical specific energy of 2600 Wh/kg are considered one of the most promising next-generation rechargeable battery systems for the electrification of transportation. This review introduces electrode manufacturing and modeling methodologies to overcome the current technical issues. The prospects for rational modeling and manufacturing strategies are discussed, to establish a new design standard for Li/S batteries.

  • lithium/sulfur cells
  • computational modeling
  • sulfur electrode
  • electrolyte to sulfur ratio
  • high specific energy

1. Introduction

Li/S batteries possess exceptional specific energy and a standard open-circuit potential of 2.15 V [14][1]. The theoretical maximum specific energy of a Li/S battery is 2600 W h kg S−1 [15][2], assuming the sulfur is fully utilized. However, complete utilization is not technically feasible due to the sluggish reaction kinetics and the insulating properties of elemental sulfur [16,17][3][4]. In addition, the weight of other cell components such as current collectors, separator, electrolyte, and cell housing materials should be considered when estimating the actual specific energy of Li/S batteries. Considering practical cell design parameters, a more realistic projection estimates the specific energy of Li/S batteries at between 400 and 500 W h kg−1 [12,18][5][6]. Though still early in the research phase of development, this technology currently exceeds the peak specific energy of Li-ion cells by about 100 W h kg−1 [12,19][5][7]. Li/S batteries also show improved safety during operation under abuse conditions compared to LiBs. Nail penetration tests demonstrated no explosive energy release when punctured due to a solid insulating coating that forms from lithium polysulfide (Li-PS), thereby preventing an internal short circuit via the penetrator [20][8]. Furthermore, because of the chemical stability of the active materials in the completely discharged state, the cells can be stored indefinitely without experiencing damage and without hazard risk [21][9].

2. Challenges in the Design and Development of Li/S Cell Components

2.1. Sulfur Electrode Design Challenges

The major technical challenges of Li/S cells discussed in the introduction section are mainly associated with the sulfur electrode. Although the sulfur electrode can theoretically deliver a very high specific capacity of 1675 mA h gS−1, the practical utilization of sulfur is still not satisfactory (generally <60%) and the long-term cyclability and power density are not yet compatible with those of Li-ion cells. The problematic Li-PS shuttle causes the loss of active sulfur and the deposition of Li2S onto the lithium electrode, resulting in cell capacity degradation. Inhomogeneous reconfiguration of the sulfur electrode microstructure due to the uncontrolled Li-PS formation and the volume changes of sulfur particles is another concern for the design of sulfur electrodes. On the other hand, Li-PS is also considered essential for Li/S-cell operation because it provides the sulfur electrodes with a kinetically fast route for reaction with lithium ions [141][10]. The solution-based pathway involving Li-PS can overcome the large energy barrier for the charge transfer reaction at the sulfur/electrolyte interface and Li-ion diffusion through solid sulfur particles. Ideally, sulfur electrodes should prohibit the Li-PS shuttle while allowing homogeneous Li-PS formation and deposition back to the sulfur electrode as sulfur or Li2S.
The sulfur–carbon composite is the most popular active material design for improving the sulfur electrode’s capacity and cyclability by mitigating the Li-PS shuttle effect. Various carbonaceous materials such as porous carbons, 1D carbons (carbon nanotubes, carbon nanofibers), and 2D carbons (graphene and graphene oxide) have been investigated as sulfur host materials or functional additives for sulfur electrodes to utilize their excellent electronic conductivity and good mechanical strength. Some carbonaceous materials have high sulfur-philic properties, which can mitigate the Li-PS shuttle phenomenon, resulting in improved cyclability [12,14,53,142,143,144][5][1][11][12][13][14]. Sulfur electrodes are manufactured by the slurry-based tape casting process like Li-ion cell electrodes, but the sulfur electrodes reported in research papers generally contain a large amount of carbon, at least 10–20 wt.% or sometimes even more than 50 wt.% (including the carbon in sulfur–carbon composite active material), to sufficiently mitigate the problems of the sulfur electrodes [14][1].
The large quantity of porous carbons often severely affects the slurry quality because carbons tend to aggregate with each other or absorb too much solvent in a slurry, making the slurry less flowable or inhomogeneous. In addition, the extremely low powder density of these carbons (e.g., Super P: 0.16 g cm−3, Ketjen black: 0.1 g cm−3) tends to increase the sulfur electrode’s thickness compared to that of the Li-ion-cell positive electrode. Casting a poorly prepared slurry often results in inhomogeneous thickness distribution, pinholes, or even delamination during the drying process. Increasing the binder content may help mitigate the mechanical damage, but it increases the ‘dead weight’ of the sulfur electrode. These issues will become more formidable as higher sulfur mass loading is required to achieve high specific energy. Therefore, more systematic design strategies are needed to design high-mass-loading sulfur electrodes. To develop thick and large-area sulfur electrodes for high energy Li/S pouch cells, functional materials or electrode fabrication parameters previously evaluated in small laboratory-scale research will need to be revisited and reoptimized.
For Li/S pouch cells to succeed in the electric transportation battery market, we anticipate that a high specific energy of >300 W h kg−1 is required. Design calculation of the obtainable specific energy (W h kg−1) for a Li/S pouch cell is helpful to predict the essential design parameters of Li/S pouch cells. In particular, we evaluated the effect of sulfur mass loading, sulfur content, E/S ratio, and sulfur utilization (equivalent to the specific capacity of the sulfur electrode) on the obtainable specific energy of Li/S pouch cells. The passive weight values of pouch cell components listed in Table 1 were used for the design calculations. Figure 4a shows the specific energy of the Li/S pouch cell for various sulfur mass loadings and E/S ratios. The results indicate that a specific energy of >300 W h kg−1 requires a high sulfur mass loading of >~6 mg cm−2 and a low E/S ratio of ≤4 while achieving >70% sulfur utilization (1172.5 mA h gS−1). A Li/S pouch cell with an E/S ratio of higher than six is unlikely to show the desired specific energy, no matter what the sulfur mass loading, unless significantly higher sulfur utilization (80–90%) is achieved. Figure 4b shows the variation of the obtainable specific energy for various sulfur utilization and sulfur mass loading values at a fixed E/S ratio of 3. The results (Figure 4b) indicate that sulfur utilization plays a critical role in achieving a high specific energy of >300 W h kg−1. The 50% utilization is still mandated even with a very high sulfur loading of 20 mg cm−2 and a low E/S ratio of 3. According to the design calculations shown in Figure 4a,b, targeting the following electrode fabrication parameters is reasonable for delivering a specific energy of 300 W h kg−1: a sulfur loading of ≥6 mg cm−2, a sulfur utilization of ≥70%, and an E/S ratio of ≤4. The design parameters become more challenging for a higher specific energy of 400 W h kg−1: a sulfur loading of ≥10 mg cm−2, a sulfur utilization of ≥80%, and an E/S ratio of ≤3. Unfortunately, a low E/S ratio of ≤4 often results in low sulfur utilization [145][15] or poor cyclability [146][16] as it results in a slow electrochemical process at the sulfur electrode (e.g., low Li-PS solubility, electrolyte decomposition) [147,148][17][18].
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Figure 4. Design calculation of Lithium-Sulfur (Li/S) pouch cell specific energy (a) Obtainable specific energy for various sulfur mass loading and electrolyte/sulfur (E/S) ratios. Sulfur content and sulfur utilization were fixed to 80% and 70%, respectively. (b) Obtainable specific energy for various sulfur loading and sulfur utilization. E/S ratio and sulfur content were fixed to 3 and 80%, respectively.
A low E/S ratio of 3–4 with reasonable specific capacity (1000–1200 mA h gS−1) has recently been reported via chemical or morphological modification of sulfur electrodes [149,150,151,152][19][20][21][22]. Although the capacity retentions were not sufficient for commercialization, the authors’ accomplishments in successfully reducing the E/S ratio emphasize the importance of the sulfur electrode design. However, in most research studies, the electrolyte amount was adjusted to achieve the target E/S ratio, regardless of the electrode’s design parameters such as porosity and sulfur content. In principle, however, the pore volume and the electrode surface area generally determine the required amount of the liquid electrolyte, since the pores of the sulfur electrode are supposed to be filled by the liquid electrolyte to form a continuous ionic percolation network and an adequate electrochemical interface. Li/S cells suffer from either electrolyte shortage or excess if the electrolyte amount is determined without considering these parameters.
For this reason, we performed additional design calculations for the Li/S pouch cell to reflect the correlation between porosity, sulfur content, and the E/S ratio. We assumed that the required electrolyte volume was the same as the total pore volume of the sulfur electrode and a separator, with no excess added. The design calculation results shown in Figure 5a,b indicate that the porosity of the sulfur electrode plays a critical role in determining the obtainable specific energy because the E/S ratio increases as the porosity of the electrode increases. We previously stated that a sulfur mass loading of >6 mg cm−2 would provide an opportunity to achieve 300 W h kg−1 when the E/S ratio is ≤4 and sulfur utilization is <70% (Figure 4a). However, according to the design calculation, the low E/S ratio of ≤4 is unlikely to be achievable with a sulfur mass loading of 6 mg/cm2 and sulfur content of 90%, unless the sulfur electrode has an extremely low electrode porosity of 10%. Based on the calculation results, we anticipate that porosity of about 3–40% with a sulfur mass loading of >10 mg cm−2 would offer an excellent opportunity to achieve a high specific energy of >300 W h/kg.
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Figure 5. Design calculation results (a) Obtainable specific energy of a Li/S pouch cell for various porosity of sulfur electrode and sulfur mass loading. Sulfur content and sulfur utilization were assumed as 90% and 70%, respectively. (b) Variation in the E/S ratio depending on the sulfur electrode porosity and sulfur mass loading. The required electrolyte amount corresponds to the pore volume of the sulfur electrode and separator. The sulfur content was assumed as 90%. (c) Variation in the E/S ratio depending on the sulfur electrode porosity and sulfur content. Sulfur mass loading used for the calculation was 10 mg cm−2.
The effect of electrode porosity raises another critical question about the influence of the sulfur content on the E/S ratio. The design calculations for the Li/S pouch cells shown in Figure 4 result in similar calculated specific energies of 311 W h kg−1 at a sulfur content of 50% and 349 W h kg−1 at a sulfur content of 90%, respectively (sulfur mass loading of 10 mg cm−2, sulfur utilization of 70%, and an E/S ratio of 3). However, lowering the sulfur content of the sulfur electrode to add more carbon additives while maintaining sulfur mass loading significantly increases the thickness of the sulfur electrode. In other words, the total pore volume of the sulfur electrode is larger when the sulfur content is lowered, implying a higher E/S ratio for the Li/S cells. The design calculation results (Figure 5c) clearly show that the sulfur content affects the E/S ratio dramatically. According to the design calculation results, an E/S ratio of <~4 for a sulfur mass loading of 10 mg cm−2 is only achievable when the sulfur content is higher than 80% and the porosity is lower than 30% for the sulfur electrode. The calculated specific energies of the Li/S pouch cells (sulfur mass loading of 10 mg cm−2 and sulfur utilization of 70%) for sulfur content values of 50% and 90% are 184.6 and 314.2 W h kg−1, respectively, which is obviously different from the calculated specific energy without consideration of porosity. The design calculation results emphasize that a high-sulfur-mass-loading electrode with a large quantity of carbon additive will not offer high specific energy even if high sulfur utilization is achieved. However, increasing the sulfur content can lower the sulfur utilization due to poor electronic conductivity [147][17]. Thus, adjusting the sulfur content must be considered thoroughly. Of course, the effect of electrode porosity can be mitigated if a carbon additive with a higher powder density is used or the powder density of carbon is increased by incorporating sulfur into the pores of the carbon.

2.2. Challenges in Lithium Metal Electrode

The lithium electrode has been investigated for high-energy lithium metal cells due to its high specific capacity and large negative potential (−3.06 vs. NHE). However, critical technical issues associated with the unstable lithium/electrolyte interface lead to concerns about the safety and reliability of Li/S cells. The repeated inhomogeneous stripping/plating processes or the formation of an unstable SEI during the cell operation causes lithium dendrite formation and growth, resulting in substantial performance degradation and cell failure [12,144,153,154,155][5][14][23][24][25]. In addition, the lithium metal electrode in the Li/S cell also suffers from the PS phenomenon. Until now, there have been various approaches to stabilizing the lithium/electrolyte interface, and these methods generally focus on the formation of a stable and ionically conductive SEI. However, despite the significant progress in improving the stability of lithium electrodes, the reliability of lithium metal electrodes is still not satisfactory for commercialization.
While enormous research efforts to stabilize the lithium metal electrode have already been made, we also need to address the challenges for Li/S cell manufacturing. According to the report by Park et al., an areal capacity of 10 mA h cm−2 results in repeated lithium stripping and plating of around 50 µm in thickness [156][26]. This result implies that the lithium/electrolyte interface of the high-capacity Li/S cell will experience dramatic volume changes during cell operation. In addition, Li/S cells may require excessive lithium, generally 1.5–2 times more than the theoretical estimation, to prevent lithium depletion. However, the optimal lithium amount needs to be determined since excessive lithium increases the material cost and the weight of the Li/S cells. The high lithium metal cost due to the expensive thin-film engineering process is another potential problem for Li/S cell commercialization. According to a review article [157][27], the price for 200 μm thickness lithium foil is about USD 100–300 per kg, whereas a 20 μm thick lithium foil costs USD 250–1000 per kg. The author pointed out that using the thick lithium metal foil may provide an opportunity to omit a copper current collector, compensating for the cell weight increase, but the absence of a copper current collector may cause an electron conduction problem for the large-area Li/S pouch cell. Lithium-free electrodes such as carbon and silicon electrodes could be alternatives to resolve lithium-metal-related issues, but they significantly lower the specific energy of Li/S cells, and the use of a Li2S electrode will be enforced, which will substantially increase the cell manufacturing cost.

2.3. Electrolyte Design Challenges

Li/S cells generally employ liquid organic electrolytes consisting of ether-based solvents, LiTFSI as a lithium salt, and LiNO3 as a functional additive. Tetraethylene glycol dimethyl ether (TEGDME) and a mixture of DOL and DME are the most widely used solvents among ether-based solvents. Ether-based solvents promote the Li-PS-based reaction route of sulfur electrodes, enabling a high sulfur utilization. Some other salts and solvents such as lithium perchlorate (LiClO4), lithium trifluoromethanesulfonate (LiCF3SO3), LiPF6, and carbonates-based solvents have also been investigated, but they are not as effective as the conventional electrolyte systems. As discussed above, the Li-PS shuttle effect easily occurs in the absence of an effective barrier. LiNO3 is an effective functional additive that mitigates the shuttle effect by forming a passivation layer on the lithium metal electrode, but recent studies indicate that LiNO3 in the liquid electrolyte can increase the consumption of the active sulfur as a result of SEI formation on the sulfur electrode [158][28], or serve as a source of gas formation at approximately 40 °C [159][29].
Organic liquid electrolytes with high salt concentrations have been investigated as an alternative solution to mitigate the Li-PS shuttle effect. Several reports suggested that the high Li-ion concentration in electrolytes affects PS solubility due to the dissolution equilibrium principle. The results showed that the higher the lithium salt concentration, the lower the Li-PS solubility; hence, the Li-PS shuttle can be prevented, improving the cyclability of Li/S cells [160][30]. Similarly, adding ionic liquids to the ether-based electrolyte also improves the cyclability of sulfur electrodes by mitigating the Li-PS shuttle [127][31] or forming a passivation surface layer on a lithium metal electrode [161][32]. However, neither high-salt-concentration electrolytes nor ionic-liquid-containing electrolytes are feasible for commercialization because they often exhibit very high viscosity that causes a high voltage polarization or low power capability. In addition, LiTFSI and ionic liquids are still relatively expensive, so they are not economical options for Li/S cell manufacturing.
Solid electrolytes for solid-state Li/S cells have recently attracted attention because they can inhibit Li-PS formation and the shuttle effect due to the lack of liquid medium. In addition, the high mechanical strength of the solid electrolytes offers a better opportunity to suppress the internal cell shorting caused by lithium dendrite growth. Moreover, the wide thermal and electrochemical windows and non-flammable natures of the solid electrolytes can dramatically reduce the safety concerns regarding the Li/S cells. However, none of the research has demonstrated high-performance solid-state Li/S cells comparable to conventional Li/S cells. Unfortunately, solid-state Li/S cells have several technical problems at the lithium–solid electrolyte and the sulfur–solid electrolyte interfaces [162,163,164][33][34][35]. Furthermore, the solid-state interfacial electrochemistry of the Li/S solid-state cells is not fully understood; thus, intensive fundamental research is certainly required. Some studies showed improved sulfur utilization and cyclability, but their testing conditions are far from those of practical operation conditions (e.g., too low discharge cut-off voltage, high operation temperature, or low current for testing).

2.4. Separator Design Challenges

Developing a polymer separator that can enhance the performance of Li/S cells is also essential because the separator can also directly impact the electrochemical behavior of Li/S cells. Polyethylene (PE) and polypropylene (PP) with the micrometer-sized pores common in LiBs [165][36] are also used in Li/S cells. More details on the separator technology are available in review articles [166,167][37][38]. The separator is one of the main contributors to the material cost for LiB manufacturing [168][39], and its weight and the electrolyte filling in its pores contributes to the weight of inactive components, implying that the separator design will be the critical factor determining the specific energy and energy cost of Li/S cells. For designing the Li/S cell separator, several things must be considered thoroughly: (1) the wettability towards ether-based electrolytes, (2) the thickness and porosity to minimize the inactive weight increase, (3) the thermal and mechanical stability, (4) the barrier functionality to prevent Li-PS shuttling, and (5) a low material price. Integration of a functional interlayer into the separator or chemical surface modification to existing PE or PP separators would also be effective. For substantial improvement of Li/S cell performance for commercialization, the above-mentioned design parameters must be considered while carefully investigating its compatibility with other cell components such as lithium and sulfur electrodes and the electrolyte.

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