Electrochemical Performance of Graphene-Modulated Sulfur Composite Cathodes Using LiBH 4 Electrolyte for All-Solid-State Li-S Battery

: All-solid-state Li-S batteries (use of solid electrolyte LiBH 4 ) were prepared using cathodes of a homogeneous mixture of graphene oxide (GO) and reduced graphene oxide (rGO) with sulfur (S) and solid electrolyte lithium borohydride (LiBH 4 ), and their electrochemical performance was reported. The use of LiBH 4 and its compatibility with Li metal permits the utilization of Li anode that improves the vitality of composite electrodes. The GO-S and rGO-S nanocomposites with different proportions have been synthesized. Their structural and morphological characterizations were performed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), and the results are presented. The electrochemical performance was tested by galvanostatic charge-discharge measurements at a 0.1 C-rate. The results presented here demonstrate the successful implementation of GO-S composites in an all-solid-state battery.


Introduction
Lithium-sulfur (Li-S) batteries have attracted attention all over the world for the future energy needs due to the material's low cost, easy availability, non-hazardous nature and high specific capacity of about 1675 mAh/g with a high theoretical specific energy density of 2600 Wh/kg [1,2]. However, Li-S batteries are progressing slowly due to polysulfide (PS) and dendrite formation, which significantly abbreviates the cycling life and raises safety issues for Li-S batteries [3][4][5][6][7][8]. To overcome these issues, the trapping of sulfur inside the porous carbon material matrix with different arrangements (for example, graphene, graphene oxide, porous carbon, and carbon nanotubes) were employed [9][10][11][12]. Out of these, GO and rGO have attracted extensive consideration [13][14][15][16][17] due to their stability, lightweight, enormous surface territory, high electrical conductivity, incredible adaptability, and mechanical properties compared to other carbon materials [18,19]. GO can easily be deposited on the surface of the material and binds it due to its hydrophilic nature as it contains more oxygen contents as compared to graphene. The rGO can be obtained from GO by washing it with hydrogen hydrate, which reduces the oxygen content in rGO as compared to GO, which makes it more conductive and is helpful to achieve better performance in Li-ion battery. The unique construction of GO-S nanocomposite electrodes improves the performance of Li-S batteries by adjusting volume expansion during electrochemical testing. Moreover, rGO, with its vast surface region alongside pervasive cavities, can set up better electronic contact with sulfur to prevent conglomeration for the smooth flow of ions through the material. Peng Yu et al. [20], in their experimental study, prepared an electrode material by reducing graphene through a solution-based technique to encapsulate sulfur inside reduced graphene sheets, which resulted in better charging-discharging of a cell with good cycle ability and capacity. Yu-Yun Hsieh et al. [21] supported that carbon materials provide a major contribution to overcoming shuttle phenomena by trapping polysulfides and also boost the cathode's conductivity. This was accomplished by synthesizing three-dimensional graphene with oxygen functionalization to form a composite with sulfur, which demonstrates excellent electrochemical performance as compared to previous works.
Different types of polymer electrolytes and composite electrolytes with perovskite and oxides are drawing attention for their application in Li-S batteries to overcome the safety issue of liquid electrolytes. In context to this, LiBH 4 -a notable hydrogen storage materialhas been studied as a promising electrolyte material for the fabrication of all-solid-state batteries. The phase transition of LiBH 4 at~115 • C provides high ionic conductivity (~1 mS/cm), which allows for its use as a solid electrolyte that supports the flow of lithium ions through it [22][23][24][25][26]. LiBH4 is safer to be used in comparison to liquid flammable electrolytes. It is quite stable up to 450 • C, after which it decomposes to LiH and B. In this work, LiBH 4 is employed with the GO-S and rGO-S composite cathode material and lithium foil as an anode. The use of GO and rGO as an additive with sulfur in this work is expected to accommodate the volume expansion as a result of the expanded surface area and cushioning effect due to the GO and rGO network.

Synthesis of GO-S and rGO-S Nanocomposites
Hummer's method [27] was used to synthesize the powdered GO, whereas rGO was obtained by chemical reduction of GO (washing of obtained GO with hydrogen hydrate to reduce oxygen content). The GO-S and rGO-S composites were synthesized via ball-milling for 1gm batch of composites that had different amounts of sulfur and GO/rGO, i.e., (i) S 99% and GO 1%, (ii) S 90% and GO 10%, (iii) S 99% and rGO 1%, and (iv) S 90% and rGO 10%. The milling was carried out for 24 h at 300 rpm using 20 stainless steel (SS) balls of 7 mm diameter.

Electrode Material Preparation
The electrode (Cathode) composite materials of GO-S and rGO-S were synthesized via ball-milling with LiBH 4 and acetylene black (AB) in 40:30:30 weight proportion for 2 h, resting for 30 min after 1 h of milling under an argon environment. For the synthesis of a 200 mg sample (80 mg active material, i.e.,S-GO/rGO composite, 60 mg solid electrolyte LiBH 4 and 60 mg AB), 10 SS balls were utilized at 370 rpm in a Fritsch P7 processing machine. LiBH 4 and AB were both dried using a dynamic vacuum at 200 • C for 24 h before utilizing them to prepare the cathode.

Coin Cell Preparation
To test the electrochemical performance of the above-mentioned electrode materials, coin cells were fabricated with Li-foil as the anode, LiBH 4 as the electrolyte, and the abovementioned composites (S 99% and GO 1%, S 90% and GO 10%, S 99% and rGO 1%, and S 90% and rGO 10% with LiBH 4 and AB) as the cathode layer. To fabricate the cell, a three-layer pellet was prepared by the following procedure. First, Li-foil (thickness of 0.14 mm) purchased from Honjo metal Co. Ltd., Osaka, Japan, in a circular shape was spread onto an SS plate as the first layer (thickness approx. 0.63 mm), then dried LiBH 4 powder (~80 mg) was sprinkled and pressed under 10 MPa for 5 min using a hydraulic press machine. It was followed by spreading composite powder (~10 mg) as the 3rd layer and was pressed under 40 MPa for 5 min. The prepared 3-layered pellet was put in a coin cell case using a perfluoroalkoxy (PFA) gasket. The samples were kept completely isolated from atmospheric conditions throughout the investigation. All the handling was conducted inside a high-purity argon-filled glove box (oxygen and moisture content <0.1 ppm) to prevent the samples from contamination of oxygen.

Material Characterization and Electrochemical Measurements
X-ray diffraction (XRD) using the Rigaku-RINT 2500 with CuKα (λ = 1.5406 Å) as the radiation source was performed for all the samples within a 2θ range of 5 • -50 • at the scan speed 5 • /min. The Debye Scherer formula was used to calculate the crystallite size D = 0.9λ/βcosθ, where λ = X-ray wavelength, θ = Bragg diffraction angle, β = full width at half maximum (FWHM).
Scanning electron microscopy (JEOL JSM-6380, JEOL Ltd., Tokyo, Japan) was used for surface morphology and energy dispersive spectroscopy (EDS) for elemental analysis. It is to note here that SEM/EDS observations were made on the top of the surface of the 3-layered pellet, which means that all the signals are from the cathode composite and not from the electrolyte layer (LiBH 4 ) or the anode layer (Li foil). A charge-discharge analyzer (HJ1001SD8, Hokuto Denko Co.) was used for the measurement of electrochemical performance of the graphene modulated GO-S and rGO-S composite cathode with LiBH 4 electrolyte at 0.1 C-rate and 120 • C temperatures. Figure 1a,b shows X-ray diffraction spectra of GO-S and rGO-S composites with acetylene black (AB) and LiBH 4 . The individual curves are also shown in the Supplementary Materials for better clarity ( Figures S1-S5). The diffraction peaks at 2θ = 22.9 • , 25.9 • , and 28.0 • , which were indexed as (222), (026), and (040) planes, respectively. These represent the orthorhombic structure (JCPDS card no. 001-0478) of sulfur with high crystallinity [18]. The graphene oxide shows diffraction peaks at 2θ = 10.9 • (001), 25.6 • (002), and 43 • (100). The GO-S and rGO-S composite with different wt% of GO and rGO showed all diffraction peaks as that for pristine sulfur, which confirms no phase transformation occurred due to GO and rGO in the composites. The detected characteristic peaks of S-GO and S-rGO composites match very well, demonstrating the orthorhombic structure. The presence of LiBH 4 was confirmed in the composite material by the presence of peaks at 2θ = 17.7 • ,23.68 • , 24.48 • , 25.24 • , 26.78 • , 28.8 • , and 40.32 • . The average crystallite size of composites was 28 nm, as calculated by the Debye Scherer formula [11].

X-ray Diffraction
Energies 2021, 14, x FOR PEER REVIEW 3 of 12 powder (~80 mg) was sprinkled and pressed under 10 MPa for 5 min using a hydraulic press machine. It was followed by spreading composite powder (~10 mg) as the 3rd layer and was pressed under 40 MPa for 5 min. The prepared 3-layered pellet was put in a coin cell case using a perfluoroalkoxy (PFA) gasket. The samples were kept completely isolated from atmospheric conditions throughout the investigation. All the handling was conducted inside a high-purity argon-filled glove box (oxygen and moisture content <0.1 ppm) to prevent the samples from contamination of oxygen.

Material Characterization andElectrochemical Measurements
X-ray diffraction (XRD) using the Rigaku-RINT 2500 with CuKα (λ = 1.5406 Å ) as the radiation source was performed for all the samples within a 2θ range of 5°-50° at the scan speed 5°/min. The Debye Scherer formula was used to calculate the crystallite size D = 0.9λ/βcosθ, where λ = X-ray wavelength, θ = Bragg diffraction angle, β = full width at half maximum (FWHM).

SEM Analysis
Energies 2021, 14, x FOR PEER REVIEW 4 of 12 AB, (v) (S100%)40%+(LiBH4)30%+(AB)30%, (vi) (S99%rGO1%)40%+(LiBH4)30%+(AB)30%, and (vii) (S90%rGO10%)40%+(LiBH4)30%+(AB)30%. Figure 2a,b shows the SEM images of the top surface of the three-layer pellet, having a cathode composite part on the top, followed by the LiBH4 layer and the Li-foil layer of pristine and cycled (30 cycles) Li/GO-S/LiBH4 systems, respectively. Several major, as well as minor, cracks have been observed in the cycled system, which might be attributed due to the volume expansion and contraction during charging and discharging of the cell. The dynamics of the system, which further affects the battery performance, can be verified from Galvanostatic charge-discharge profiles. The system showed good capacity initially; however, the performance was degraded with the beginning of crack formation.   Figure 2a), from which the uniform distribution of sulfur, carbon, and boron was seen with mapping percentages of 72.9%, 16.1%, and 6.1%, respectively, as calculated from Figure 3e. However, EDS spectra reflect Li in very small quantities in comparison to the expected content from LiBH4, which is due to the low energy of characteristic radiation of Li.    Figure 2a), from which the uniform distribution of sulfur, carbon, and boron was seen with mapping percentages of 72.9%, 16.1%, and 6.1%, respectively, as calculated from Figure 3e. However, EDS spectra reflect Li in very small quantities in comparison to the expected content from LiBH 4 , which is due to the low energy of characteristic radiation of Li.

SEM Analysis
Energies 2021, 14, x FOR PEER REVIEW 4 of 12 AB, (v) (S100%)40%+(LiBH4)30%+(AB)30%, (vi) (S99%rGO1%)40%+(LiBH4)30%+(AB)30%, and (vii) (S90%rGO10%)40%+(LiBH4)30%+(AB)30%. Figure 2a,b shows the SEM images of the top surface of the three-layer pellet, having a cathode composite part on the top, followed by the LiBH4 layer and the Li-foil layer of pristine and cycled (30 cycles) Li/GO-S/LiBH4 systems, respectively. Several major, as well as minor, cracks have been observed in the cycled system, which might be attributed due to the volume expansion and contraction during charging and discharging of the cell. The dynamics of the system, which further affects the battery performance, can be verified from Galvanostatic charge-discharge profiles. The system showed good capacity initially; however, the performance was degraded with the beginning of crack formation.   Figure 2a), from which the uniform distribution of sulfur, carbon, and boron was seen with mapping percentages of 72.9%, 16.1%, and 6.1%, respectively, as calculated from Figure 3e. However, EDS spectra reflect Li in very small quantities in comparison to the expected content from LiBH4, which is due to the low energy of characteristic radiation of Li.   Figure 4a,b shows SEM images of the top surface of the 3-layer pellet, having the cathode composite part on the top, followed by the LiBH 4 layer and the Li-foil layer of pristine and cycled (42 cycles) Li/rGO-S/LiBH 4 systems, respectively. Although there is no direct evidence as SEM analysis was performed only after several cycles, by combining the SEM analysis and the cyclic charge-discharge profiles, it can be speculated that the majority of crack formation occurs during the initial cycling of the cell (the cell initially delivered better performance, which started degrading immediately as the crack formation occurred). On further cycling, these cracks started expanding more, thus resulting in fast capacity decay, as seen from electrochemical testing. These cracks affect the flow of Li-ions through the electrodes. As compared to the GO-S electrode, the rGO-S electrode exhibits less crack formation and suggests that the electrode may benefit in terms of the capacity and cycling performance of a battery. The addition of any carbon material is to accommodate the volume expansion and act as a binder for sulfur. The smaller cracks in rGO are due to the fact that it has a larger surface area than GO, so it can bind the electrode material (sulfur) more efficiently compared to GO.

SEM Analysis
Energies 2021, 14, x FOR PEER REVIEW 5 of 12 Figure 4a,b shows SEM images of the top surface of the 3-layer pellet, having the cathode composite part on the top, followed by the LiBH4 layer and the Li-foil layer of pristine and cycled (42 cycles) Li/rGO-S/LiBH4 systems, respectively. Although there is no direct evidence as SEM analysis was performed only after several cycles, by combining the SEM analysis and the cyclic charge-discharge profiles, it can be speculated that the majority of crack formation occurs during the initial cycling of the cell (the cell initially delivered better performance, which started degrading immediately as the crack formation occurred). On further cycling, these cracks started expanding more, thus resulting in fast capacity decay, as seen from electrochemical testing. These cracks affect the flow of Li-ions through the electrodes. As compared to the GO-S electrode, the rGO-S electrode exhibits less crack formation and suggests that the electrode may benefit in terms of the capacity and cycling performance of a battery. The addition of any carbon material is to accommodate the volume expansion and act as a binder for sulfur. The smaller cracks in rGO are due to the fact that it has a larger surface area than GO, so it can bind the electrode material (sulfur) more efficiently compared to GO. EDS images of the rGO-S composite system (Figure 5a-d) also show the uniform distribution of S, C, and B, having mapping percentages of 70.8%, 17.4%, and 6.5%, respectively ( Figure 5e).The SEM image of the same composite electrode without cycling is shown in Figure 4a.From the mapping percentage of the composites, it can be seen that the percentage of carbon in the cathode of the rGO-S system is more when compared to the GO-S cathode system, which may be due to more homogenous mixing of the carbon content that might have played an important role in cycling, as it increased the storage capacity of the electrode. EDS images of the rGO-S composite system (Figure 5a-d) also show the uniform distribution of S, C, and B, having mapping percentages of 70.8%, 17.4%, and 6.5%, respectively ( Figure 5e).The SEM image of the same composite electrode without cycling is shown in Figure 4a.From the mapping percentage of the composites, it can be seen that the percentage of carbon in the cathode of the rGO-S system is more when compared to the GO-S cathode system, which may be due to more homogenous mixing of the carbon content that might have played an important role in cycling, as it increased the storage capacity of the electrode.  Figure 6a,b shows the Galvanostatic charge-discharge profiles of two GO-S composite electrodes with 1%GO-99%S and 10%GO-90%S as active materials, which were performed in the potential range 0.2 V to 4 V at a C-rate of 0.1 C-rate and temperature 120 °C. The capacities shown here are calculated with respect to sulfur content. It is clear that the first discharge cycle of 1%GO-99%S composite delivered a capacity of 1100 mAh/g, which is lower than the theoretical capacity. Conversely, during the charging cycle, it showed a capacity of 1700 mAh/g. This must be due to the simultaneous thermochemical reaction between S and LiBH4 (present as a component in the electrode layer), as suggested in our previous works on M2S3 (M = Bi and Sb) [28][29][30][31]. Actually, during the initial heating process, sulfur partially reacted with LiBH4 (present as a component in the electrode layer) and formed Li2S thermo chemically. Then, during the discharge cycle, the remaining sulfur further reacted with Li-ions (coming from Li-foil at the anode side) electrochemically and converted into Li2S. This is the reason why the initial capacity was observed lower than the theoretical capacity. When it comes to the charging cycle, Li2S starts releasing Li-ions and forms S again. However, this freshly formed S reacts with LiBH4 (present as a component in the electrode layer) thermo chemically again and is converted into Li2S. At this time, the charging process is also going on, so the thermo chemically formed Li2S keeps releasing Li-ions. These simultaneous reactions of S-LiBH4 (thermo chemical) and the Li2S to S conversion (electrochemical charging) continue until the consumption of LiBH4. During the first charging cycle, the cell delivers a capacity more than the theoretical (1675mAh/g) capacity during charging, which might be due to the reaction between sulfur with solid electrolyte LiBH4 (present as a component in the electrode layer) acting as a Li source. The capacity in the first charging cycle was observed higher than the corresponding discharge cycle. A similar behavior was also observed for the other composite (Figure 6b).The first cycle suggests a coulombic efficiency (CE) ~65% for the 1%GO-99%S composite, whereas the cell that has GO10%S90%composite material showed ~84% coulombic efficiency. It can be seen that CE is less, which resembles the parasitic side reaction occurring between S and LiBH4 and irregular Li-ion movements as a result of the shapes and values of the cell voltages' plateau changing dramatically in subsequent cycles. The discharge reaction of the cell was confirmed by comparing the XRD profile before and after the discharge cycle ( Figure S6), where the peaks corresponding to sulfur are found in the as-prepared cell. In addition to sulfur peaks, small peaks corresponding to the Li2S phase are also observed, which confirms our  Figure 6a,b shows the Galvanostatic charge-discharge profiles of two GO-S composite electrodes with 1%GO-99%S and 10%GO-90%S as active materials, which were performed in the potential range 0.2 V to 4 V at a C-rate of 0.1 C-rate and temperature 120 • C. The capacities shown here are calculated with respect to sulfur content. It is clear that the first discharge cycle of 1%GO-99%S composite delivered a capacity of 1100 mAh/g, which is lower than the theoretical capacity. Conversely, during the charging cycle, it showed a capacity of 1700 mAh/g. This must be due to the simultaneous thermochemical reaction between S and LiBH 4 (present as a component in the electrode layer), as suggested in our previous works on M 2 S 3 (M = Bi and Sb) [28][29][30][31]. Actually, during the initial heating process, sulfur partially reacted with LiBH 4 (present as a component in the electrode layer) and formed Li 2 S thermo chemically. Then, during the discharge cycle, the remaining sulfur further reacted with Li-ions (coming from Li-foil at the anode side) electrochemically and converted into Li 2 S. This is the reason why the initial capacity was observed lower than the theoretical capacity. When it comes to the charging cycle, Li 2 S starts releasing Li-ions and forms S again. However, this freshly formed S reacts with LiBH 4 (present as a component in the electrode layer) thermo chemically again and is converted into Li 2 S. At this time, the charging process is also going on, so the thermo chemically formed Li 2 S keeps releasing Li-ions. These simultaneous reactions of S-LiBH 4 (thermo chemical) and the Li 2 S to S conversion (electrochemical charging) continue until the consumption of LiBH 4 . During the first charging cycle, the cell delivers a capacity more than the theoretical (1675mAh/g) capacity during charging, which might be due to the reaction between sulfur with solid electrolyte LiBH 4 (present as a component in the electrode layer) acting as a Li source. The capacity in the first charging cycle was observed higher than the corresponding discharge cycle. A similar behavior was also observed for the other composite (Figure 6b).The first cycle suggests a coulombic efficiency (CE)~65% for the 1%GO-99%S composite, whereas the cell that has GO 10% S 90% composite material showed~84% coulombic efficiency. It can be seen that CE is less, which resembles the parasitic side reaction occurring between S and LiBH 4 and irregular Li-ion movements as a result of the shapes and values of the cell voltages' plateau changing dramatically in subsequent cycles. The discharge reaction of the cell was confirmed by comparing the XRD profile before and after the discharge cycle ( Figure S6), where the peaks corresponding to sulfur are found in the as-prepared cell. In addition to sulfur peaks, small peaks corresponding to the Li 2 S phase are also observed, which confirms our hypothesis of a reaction between S and LiBH 4 during the initial heating. These peaks corresponding to Li 2 S becoming stronger after discharging for both the composites, and all the peaks corresponding to sulfur phase disappear. hypothesis of a reaction between S and LiBH4 during the initial heating. These peaks corresponding to Li2S becoming stronger after discharging for both the composites, and all the peaks corresponding to sulfur phase disappear.  Figure 7a shows the cycling performance of the Li-S cell investigated at a 0.1 C-rate for 30 cycles with a cut-off voltage between 0.2 V and 4.0 V. The reduction in discharge capacity was observed for both composites. The cell with the 1%GO-99%S composite showed a lower initial capacity in comparison to the other composite (10%GO-90%S), but it could be stable and could work up to 30 cycles. The cell with 10%GO-90%S composite stopped working after 16 cycles but delivered a higher capacity of around 400 mAh/g. Figure 7b shows the cyclic voltammetry of the full Li/S cell scanned at 0.1 mV/s operated between 0.2 V to 4 V, which confirms the reversibility of the reaction and is used to decide the potential window for charging-discharging cycles. The CV curve shows one oxidation peak during discharge at 0.86 V, which corresponds to S to Li2S conversion. In contrast, the curve during reduction shows two peaks, which is unusual. It can be explained on the basis of a thermochemical reaction that generated two different species of sulfur/Li2S (structurally same, but kinetically different). Thus, the splitting in the CV curve is due to the different kinetics associated with these. Figure 6. Galvanostatic charge-discharge profiles of (a) the S 99% GO 1% electrode and (b) the S 90% GO 10% electrode. Figure 7a shows the cycling performance of the Li-S cell investigated at a 0.1 C-rate for 30 cycles with a cut-off voltage between 0.2 V and 4.0 V. The reduction in discharge capacity was observed for both composites. The cell with the 1%GO-99%S composite showed a lower initial capacity in comparison to the other composite (10%GO-90%S), but it could be stable and could work up to 30 cycles. The cell with 10%GO-90%S composite stopped working after 16 cycles but delivered a higher capacity of around 400 mAh/g. Figure 7b shows the cyclic voltammetry of the full Li/S cell scanned at 0.1 mV/s operated between 0.2 V to 4 V, which confirms the reversibility of the reaction and is used to decide the potential window for charging-discharging cycles. The CV curve shows one oxidation peak during discharge at 0.86 V, which corresponds to S to Li 2 S conversion. In contrast, the curve during reduction shows two peaks, which is unusual. It can be explained on the basis of a thermochemical reaction that generated two different species of sulfur/Li 2 S (structurally same, but kinetically different). Thus, the splitting in the CV curve is due to the different kinetics associated with these. Figure 8a,b shows galvanostatic charge-discharge profiles of Li/S cells containing 1%rGO-99%S and 10%rGO-90%S composite electrodes in the potential range of 0.2 V to 4.0 V at a 0.1 C-rate and 120 • C temperature. The cell with the1%rGO-99%S composite material showed a discharge capacity of 1104 mAh/g (lower than the theoretical capacity) and a charge capacity of 1698 mAh/g (higher than the discharge capacity) in the initial cycle with~65% coulombic efficiency, which reduced to a constant value around 150 mAh/g after 42 cycles (Figure 8a). The cell with 10%rGO-90%S composite material showed an initial charge capacity of 1309 mAh/g and a discharge capacity of 1165 mAh/g with añ 89% coulombic efficiency. The behavior is quite similar to that of the GO-added samples but with a better capacity. Besides these findings, the second discharge cycle in all the cases showed a higher discharge capacity, which is also associated with the thermochemical reaction between S and LiBH 4 . In the first cycle, most of the LiBH 4 is already exhausted, which leaves more sulfur unreacted and is available to participate in the electrochemical reaction with Li-ions (see above-initially, sulfur reacted with LiBH 4 during heating, which reduced the available sulfur content to participate in electrochemical cycling). cide the potential window for charging-discharging cycles. The CV curve shows one oxidation peak during discharge at 0.86 V, which corresponds to S to Li2S conversion. In contrast, the curve during reduction shows two peaks, which is unusual. It can be explained on the basis of a thermochemical reaction that generated two different species of sulfur/Li2S (structurally same, but kinetically different). Thus, the splitting in the CV curve is due to the different kinetics associated with these.  Figure 8a,b shows galvanostatic charge-discharge profiles of Li/S cells containing 1%rGO-99%S and 10%rGO-90%S composite electrodes in the potential range of 0.2 V to 4.0 V at a 0.1 C-rate and 120°C temperature. The cell with the1%rGO-99%S composite material showed a discharge capacity of 1104 mAh/g (lower than the theoretical capacity) and a charge capacity of 1698 mAh/g (higher than the discharge capacity) in the initial cycle with ~65% coulombic efficiency, which reduced to a constant value around 150 mAh/g after 42 cycles (Figure 8a). The cell with 10%rGO-90%S composite material showed an initial charge capacity of 1309 mAh/g and a discharge capacity of 1165 mAh/g with an ~89% coulombic efficiency. The behavior is quite similar to that of the GO-added samples but with a better capacity. Besides these findings, the second discharge cycle in all the cases showed a higher discharge capacity, which is also associated with the thermochemical reaction between S and LiBH4. In the first cycle, most of the LiBH4 is already exhausted, which leaves more sulfur unreacted and is available to participate in the electrochemical reaction with Li-ions (see above-initially, sulfur reacted with LiBH4 during heating, which reduced the available sulfur content to participate in electrochemical cycling). The specific discharge capacity of cells prepared with GO-S and rGO-S composite cathode material showed competitiveness with earlier reported work in which researchers utilized different carbon composite with sulfur by adopting different procedures for composite making and electrolyte. Table 1 shows a list of a few composite materials and their initial performance compared to the results of the present work. However, from Table 1, it can be seen that innovative work of the Li-S battery is, as of yet, incipient and shows that lots of exertion are needed for the commercialization of the battery.  The specific discharge capacity of cells prepared with GO-S and rGO-S composite cathode material showed competitiveness with earlier reported work in which researchers utilized different carbon composite with sulfur by adopting different procedures for composite making and electrolyte. Table 1 shows a list of a few composite materials and their initial performance compared to the results of the present work. However, from Table 1, it can be seen that innovative work of the Li-S battery is, as of yet, incipient and shows that lots of exertion are needed for the commercialization of the battery. Figure 9a shows the cyclic performance of the Li-S coin cell containing the rGO-S composite electrodes within a potential range of 0.2 V to 4.0 V at a 0.1 C-rate and 120 • C. It has been observed that the cycling and stability of composite having 1%rGO-99%S is better than that of composite having 10%rGO-90%S. It is also noted here that the addition of both GO and rGO is helpful to obtain better stability in comparison to pure sulfur ( Figure S7), where the battery works only for two cycles.  Sulfur-reducing CV peaks at~3.33 V,~2.81 V, and~2.27 V can be seen in the reduction scan, which is very similar to that of the GO-containing composites. In the anodic scan from 0.2 V to 4.0 V, two oxidative peaks at~0.88 V and a smaller peak at~2.08 V can be seen and connected to the oxidation of PS to S. The appearance of two peaks affirms the overlapping of peaks. Figure 9a shows the cyclic performance of the Li-S coin cell containing the rGO-S composite electrodes within a potential range of 0.2 V to 4.0 V at a 0.1 C-rate and 120°C. It has been observed that the cycling and stability of composite having 1%rGO-99%S is better than that of composite having 10%rGO-90%S. It is also noted here that the addition of both GO and rGO is helpful to obtain better stability in comparison to pure sulfur ( Figure S7), where the battery works only for two cycles.  Sulfur-reducing CV peaks at ~3.33 V, ~2.81 V, and ~2.27 V can be seen in the reduction scan, which is very similar to that of the GO-containing composites. In the anodic scan from 0.2 V to 4.0 V, two oxidative peaks at ~0.88 V and a smaller peak at ~2.08 V can be seen and connected to the oxidation of PS to S. The appearance of two peaks affirms the overlapping of peaks.

Conclusions
All-solid-state Li-S batteries using two different composites of sulfur with GO and rGO were prepared, and their electrochemical performance was investigated. The mechanism of electrochemical charging/discharging is proposed herein by bringing several experiments together. This work suggests a successful implementation of sulfur in all-solid-state batteries using LiBH4 as a solid electrolyte. However, it is observed that sulfur also reacts with LiBH 4 thermochemically, which negatively impacts the stability. It is important to examine the performance at lower temperatures (<80 • C) to avoid the thermochemical reaction between sulfur and LiBH 4 . This can be achieved by using a LiBH 4 composite with high conductivity at low temperatures. Since the addition of graphene-modulated additives provided a better path for electron movements, it significantly enhanced the performance of the all-solid-state batteries studied in this work in comparison to pure S, as the sulfur cell without additives did not work for more than two cycles. From the results, it is concluded that the cycling ability and capacity are better with a cathode composite material with rGO (42 cycles) compared to GO (30 cycles) and pristine sulfur (two cycles). The treated composites have shown a great ion transport network with an expanded surface area due to the addition of GO and rGO, which counter the volume expansion of the material during charge and discharge. The results provide insight into a new composite material for enhancing the performance of the Li-S battery.