Electroconductive Additives for High-Rate Capability of High-Areal-Capacity Lithium – Sulfur Batteries Using Metal-Foam Current Collector

Various types of electroconductive additives were evaluated for high C-rate capability in an attempt to extend practical application of high-areal-capacity lithium–sulfur batteries that employ an aluminum-foam current collector. Carbon nanofibers (CNFs) were found to be the most effective additive, with the ability to attain a high-sulfur-loading of 40 mg cm−2. A CNF-containing cell exhibited gravimetric capacities of 1094 and 758 mAh gsulfur−1 (46.8 and 32.4 mAh cm−2) at 0.05 and 0.1 C-rate, respectively, in an ether-based electrolyte. Because a CNF-containing slurry exhibits low viscosity even at a high solid ratio, it could be filled into the aluminum foam. Additionally, a lithium–sulfur battery with high-sulfur-loading had an energy density of ~120 Wh kg−1, a value that was calculated from the weight of the components of the cathode, anode, current collectors, electrolyte, and separator. Assuming that the amount of electrolyte decreases and that the energy density of cells accumulate, a theoretical energy density of 522 Wh kg−1 was estimated. Moreover, it was found that even if a high-areal-capacity was achieved, the discharge capacity converged at a high C-rate, unless there was an improvement in ion diffusion in the bulk electrolyte. This is considered a limitation of sulfur cathodes with high-sulfur-loading.


Introduction
Since the practical realization of lithium-ion batteries (LIBs) in 1991 1 , their application range has grown wider and wider because of the continuous increase in their energy density.Although LIBs were used in small electronic devices such as cellular phones, video cameras, and laptop computers in the early days, they are currently widely used in electric vehicles, and stationary batteries are now capable of storing power generated by solar and wind sources.The wider application of LIBs requires them to be safe, low-cost, and in possession of high energy density.Sulfur is one of the promising cathode materials to help LIBs meet these requirements because it has low toxicity, is produced as an industrial byproduct, is abundant in nature 2 , and has high theoretical capacity (1675 mAh g −1 ) 3,4 .Although the operating potential of sulfur is low, its high theoretical capacity sufficiently compensates for the shortcoming.Owing to the high theoretical capacity of sulfur, lithium-sulfur batteries are expected to have a theoretical energy density of 2500 Wh kg −1 (or 2800 Wh L −1 ) 2,3 , which is very high when compared with that of 387 Wh kg −1 for conventional graphite-LiCoO2 batteries.However, some barriers, such as low ionic and electrical conductivity, large volume expansion upon lithiation from S8 to Li2S (approximately 180%), solubility of polysulfides, Li2Sx (4  x  8), into the electrolyte solution 5 , and low sulfur loading in the cathode 6 , hinder its practical use.The dissolution of polysulfides in the electrolyte solution results in low charge-discharge efficiency (known as the shuttle effect), self-discharge, and capacity decay.Nazar and co-workers proposed a solution to overcome the problems associated with a sulfur cathode, namely using an ordered mesoporous carbon material as a host for sulfur to improve the low ionic and electrical conductivity and to prevent polysulfide dissolution 3 .
Many studies have been conducted to tackle the problem of polysulfide dissolution by using carbon materials to physically or chemically (or both physically and chemically) adsorb the polysulfides 3,7 , by using ionic-conductive polymers to suppress migration of the polysulfides [8][9][10] , and by using semi-conductive or metallic oxides to adsorb the polysulfides owing to their high polarity 11,12 .Other approaches to tackle the polysulfide dissolution include using electrolytes such as glymebased ionic liquids 13 , which shows sparingly solubility against the polysulfides 14 , and by using an additive such as LiNO3, which forms a protective layer on the lithium metal anode to suppress the polysulfide reduction on the anode, impeding the shuttle effect 15 .
As described above, sulfur cathodes have been progressing toward practical application.As their development advanced, the low sulfur loading in the cathode and low sulfur loading ratio in the entire cathode have emerged as limiting factors 6 .The problem of low sulfur loading has been emphasized by the revelation by Hagen et al. that the areal sulfur loading of most sulfur cathodes is less than 2.0 mg cm −2 16 .Meanwhile, Gao et al. pointed out that if a lithium-sulfur battery has a sulfur loading ratio in a cathode of less than ~70%, its volumetric energy density will be lower than that of a conventional lithium-ion battery 17 .As a means to increase the sulfur loading in a cathode, Zhou et al. reported promising results by using graphene foam as the current collector; they also investigated the relationship between the sulfur loading and areal capacity 6 , reporting a high sulfur loading of 10.1 mg cm −2 with corresponding high areal capacity of 13.4 mAh cm −2 .For practical application of this approach, however, graphene foam must be produced on a large scale.Furthermore, Chen et al. reported a high sulfur loading of 15.6 mg cm −2 with corresponding high areal capacity of 8.6 mAh cm −2 by using Ti4O7 nanoparticle-embedded porous carbon host of sulfur on a nickel foam current collector. 18n our recent work, we demonstrated an unprecedented high areal capacity of 21.9 mAh cm −2 with 0.01 C-rate for discharge by using an aluminum-foam current collector and a binder consisting of carboxymethyl cellulose (CMC) and styrene butadiene rubber (SBR) 19 .To attain such a high areal capacity, it is important to fill the aluminum foam with a high-solid-ratio slurry.In the study presented here, the areal capacity and rate capability of a sulfur cathode with an aluminum-foam current collector were improved by selecting appropriate the electroconductive material, whose manufacturing process is established (commercially available), and electrolyte.The possibility to enhance the energy density of the lithium-sulfur battery was also investigated.

Results and discussion
Pieces of aluminum foam with high sulfur loading were prepared by filling them with slurries composed of the S-KB composite, CMC+SBR binder, and different electroconductive additives: AB, KB, CNTs, CNHs, or CNFs as commercially available carbons.Among the various combinations, the slurry containing CNFs had a viscosity that was sufficiently low for it to be filled into the aluminum foam at a relatively high solid ratio because of its small specific surface area, as summarized in Table 1; therefore, a S-KB based cathode with high sulfur loading in the aluminum foam was successfully obtained.The viscosities of the slurries with high solid ratio, which consisted of the other electroconductive additives, were too high, and these slurries could not be properly filled into the aluminum foam.As examples, Table 2 shows the viscosities of slurries with different solid content and containing KB or CNF as the electroconductive additives.Next, we investigated the C-rate capability of the highly loaded sulfur cathodes with different electroconductive additives-AB, KB, CNTs, CNHs, and CNFs.Among the five electroconductive additives, KB and CNFs had the largest effect on the sulfur loading and exhibited better C-rate capability.Fig. 1 illustrates typical charge-discharge curves obtained at various C-rates for the cells composed of KB and G1+G3 (i.e., the KB-(G1+G3) cell), CNFs and G1+G3 (i.e., the CNF-(G1+G3) cell), and CNFs and DOL+DME (i.e., the CNF-(DOL+DME) cell) as the electroconductive additive and electrolyte, respectively.For the discharge curves of the KB-(G1+G3) cell with sulfur loading of 15 mg cm −2 , the following behavior, which is typical of a sulfur cathode, was confirmed 3,20 : S8 was converted to Li2Sx (where x = 3-8) at ~2.10 V, and Li2Sx was converted to Li2S at ~2.05 V.The cell delivered gravimetric capacities of 1354 and 1251 milliampere hour per gram of sulfur (mAh gsulfur −1 ) at C-rates of 0.01 and 0.02 C, which are equivalent to areal capacities of 21.3 and 19.7 mAh cm −2 , respectively.For the discharge at higher C-rate, the second plateau that represents the conversion of Li2Sx to Li2S was hardly confirmed.On the other hand, for the discharge curves of the CNF-(G1+G3) cell with sulfur loading of 20 mg cm −2 , even though the typical behavior of a sulfur cathode was confirmed at a C-rate below 0.0075 C with a discharge capacity of 1255 mAh gsulfur −1 (29 mAh cm −2 ), the cells almost did not work at 0.01 C-rate.The difference is attributed to the ionic path in the sulfur cathode.It is well known that KB has a hollow structure, which allows it to retain a large amount of the liquid electrolyte.Therefore, KB enhanced the ionic conductivity in the sulfur cathode, reducing its overvoltage because of the ionic path in the sulfur cathode.Such high overvoltage can be reduced by a highly ionically conductive electrolyte.The discharge curves of the CNF-(DOL+DME) cell with sulfur loading of 20 mg cm −2 is shown in Fig. 1(c).Owing to the high ionic conductivity of the DOL+DME electrolyte, the typical behavior of a sulfur cathode was clearly observed even at 0.2 Crate, with a discharge capacity of 656 mAh gsulfur −1 (13.8 mAh cm −2 ).The plots of gravimetric discharge capacity versus the C-rate in Fig. 2(a) and (b) were converted into the plots of the areal discharge capacity versus the C-rate in Fig. 2(c) and (d).For all of the KB-(G1+G3), CNF-(G1+G3), and CNF-(DOL+DME) cells, as the sulfur loading in the cathode was increased, the areal capacity increased at lower C-rates (e.g., 0.02 C, 0.005 C, and 0.05 C).Interestingly, the areal discharge capacity converged to certain values as the C-rate increased.Consequently, the areal discharge capacities of the KB-(G1+G3) cells converged to about 1 mAh cm −2 at 1 C-rate, the areal discharge capacities of the CNF-(G1+G3) cells converged to about 4 mAh cm −2 at 0.01 C-rate, and the areal discharge capacities of the CNF-(DOL+DME) cells converged to about 0.1 mAh cm −2 at 2 C-rate.However, a precise comparison of the values of areal capacity among the same systems and at the same C-rates showed that the areal discharge capacities were higher for cells with lower sulfur loading.This shows that the limitation for the high sulfur loading cathode at high C-rate operation was caused by ion diffusion in the bulk electrolyte.Ion diffusion in the pores of the sulfur cathode also limited operation at high C-rate.The limiting current densities, which were roughly estimated from the converged areal discharge capacities and their C-rate were 1, 0.04, and 0.2 mA cm −2 for the KB-(G1+G3), CNF-(G1+G3), and CNF-(DOL+DME) systems, respectively.Even in the same solvent systems of KB-(G1+G3) and CNF-(G1+G3), rough estimates of the limiting current densities were different, which suggest that KB improved ion diffusion in the pores of the sulfur cathode owing to its hollow structure, resulting in a higher current density than that of the CNF system.In addition, in the same electroconductive additive systems of CNF-(G1+G3) and CNF-(DOL+DME), the limiting current density of the CNF-(DOL+DME) system was higher than that of the CNF-(G1+G3) system owing to the low viscosity of the DOL+DME electrolyte system.Thus, ion diffusion in the pores of the sulfur cathode account for one of the bottle-necks.However, ion diffusion in the bulk electrolyte was the principal bottle-neck, as confirmed by the convergence of the limiting current with increasing C-rate.Fig. 3 summarizes the gravimetric energy density obtained from single-layered full cells with the same amount of electrolyte as 260% of the theoretical vacancy of pores in the separator and the cathode layers.The gravimetric energy density was calculated from the weight of components of the cathode, anode, current collectors, electrolyte, and separator.The calculated values are represented by solid symbols in the figure.High sulfur loadings above 20 mg cm −2 were not obtained for the electroconductive materials of AB, CNTs, and CNHs because of the viscosities of their slurries were too high for filling into the aluminum foam.Although the KB-containing slurry was also high viscous for filling into the aluminum foam, it was possible for the aluminum foam to be filled with the slurry to the sulfur loading of 20 mg cm −2 with effort.Consequently, the electroconductive materials AB, CNTs, and CNHs could not improve the ionic diffusion in the cathode layer, resulting in very low gravimetric energy densities when they were used with glyme-based electrolytes.Because the sulfur loading of the cathodes containing AB, CNTs, and CNHs as additives was insufficient to obtain high areal capacity, the effect of the electrolytes was not evaluated.These sulfur cathodes did not succeed in enhancing the energy density probably because of inadequate formation of electron paths and iondiffusion paths.The KB-containing sulfur cathode was able to work to enhance the energy density even when a glyme-based electrolyte was used because of the well-formed ion-diffusion paths.As mentioned above, a CNF-containing sulfur cathode with high sulfur loading was successfully obtained; however, the aluminum foam with thickness of 1 mm was saturated at a sulfur loading of ~20 mg cm −2 .Therefore, aluminum foams with thickness of 1.4 mm (plotted as triangles) and 2.0 mm (plotted as squares) were filled with the appropriate slurries to obtain sulfur loadings of ~30 and ~40 mg cm −2 , respectively.The C-rate capability of the CNF-containing sulfur cathode in the glyme-based electrolyte was lower than that of the KB-containing sulfur cathode.Therefore, the ether-based electrolyte was used for the CNF-containing sulfur cathode.Because of its high sulfur loading, energy densities of ~120 Wh kg −1 were obtained for sulfur loadings of ~30 and ~40 mg cm −2 .According to our preliminary survey, the amount of electrolyte could be reduced to the volume of 100% of the theoretical vacancies for pores in the separator and cathode layers, without a noticeable decrease in charge-discharge characteristics.Therefore, the estimated energy densities, which were calculated based on the assumption that the amount of electrolyte was reduced from the amount for 260% of the pores to that for 100% of the pores, are also represented by dashed symbols in Fig. 3. Consequently, a single-layered lithium-sulfur battery with energy densities of 360 and 392 Wh kg −1 were estimated for sulfur loadings of ~30 and ~40 mg cm −2 , respectively.Furthermore, we expect to be able to obtain an estimated energy density of 522 Wh kg −1 from the sum of the energy densities of a lithium-sulfur battery with six layers of active materials.
The viscosity of the slurries was measured using a viscosity meter (VM-100A, Sekonic Holdings Corporation, Japan).
The charge-discharge properties of the sulfur cathodes were evaluated with a cut-off voltage of 1.0-3.3V, using a charge-discharge system (HJ1010SD8, Hokuto Denko Corp., Japan).

Conclusions
Various electroconductive additives, which are commercially available carbons, and electrolytes were evaluated in an attempt to improve the energy density and the C-rate capability of lithiumsulfur batteries consisting of a sulfur cathode and an aluminum-foam current collector.The slurry containing CNFs as an electroconductive additive exhibited low viscosity even at high solid ratio, attaining a sulfur loading of up to 40 mg cm −2 when injected into aluminum foam.Even for such high sulfur loading, relatively high C-rate capability was obtained by using an ether-based electrolyte, DOL+DME.For the sulfur cathode with a sulfur loading of 40 mg cm −2 , gravimetric capacities of 1094 and 758 mAh gsulfur −1 (46.8 and 32.4 mAh cm −2 ) were obtained at 0.05 and 0.1 C-rate, respectively.In addition, the lithium-sulfur battery with high sulfur loading exhibited an energy density of ~120 Wh kg −1 , which was calculated from the weight of the components of the cathode, anode, current collectors, electrolyte, and separator.By assuming that the amount of electrolyte can reduced from 260% of the pore volume to 100% of the pore volume, and by assuming that a six-layered lithiumsulfur battery has the accumulated density of six single-layered batteries, a theoretical energy density of 522 Wh kg −1 was obtained.Meanwhile, it was seen that even when high areal capacity was reached, the discharge capacity converged at a high C-rate unless there was an improvement in ion diffusion in the bulk electrolyte.

Figure 3 .
Figure 3. Gravimetric energy density obtained experimentally (solid symbols) and estimated computationally from the experimental data (dashed symbols) of the cells composed of various electroconductive additives and electrolytes as functions of sulfur loading.The cells used to obtain the experimental energy density contained 260% of the theoretical vacancy of electrolytes.To calculate the estimated energy density, the amount of the electrolyte in each cell was assumed to be 100% of the theoretical vacancy.The weight used to calculate the energy density included the weight of the cathode anode, current collectors, electrolyte, and separator.

Table 1 .
Specific surface area of the electroconductive additives.

Table 2 .
Viscosity of slurries with different solid content and containing KB or CNFs as the electroconductive additive.