Transition Metal Carbides Filler-Reinforced Composite Polymer Electrolyte for Solid-State Lithium-Sulfur Batteries at Room Temperature: Breakthrough

: All solid-state room-temperature lithium-sulfur (Li-S) batteries have gained increasing attention due to their ability to eliminate the polysulﬁdes shuttle effects and the safety dangers associated with the liquid electrolytes. Herein, a novel composite solid-state electrolyte, which is nickel-tungsten carbides (NiWC) over mesoporous silica (SBA-15) ﬁlled polyethylene oxide (PEO), was developed and investigated for Li-S batteries. The ﬁller minimizes the crystallinity of the PEO and increases the ionic conductivity of the electrolyte, resulting in lowering the AC impedance of electrolyte composite from 26,256 ohm to 2416 ohm and to 5734 ohm after adding the electrolyte material with Ni/W ratios of 1:1 and 9:1, respectively. A high initial speciﬁc capacity of 1305 mAh g − 1 and a capacity retention of 66.7% after 8 cycles at C/10 was obtained at room temperature after adding NiWC/SBA-15 with a Ni/W ratio of 1:1. This novel composite solid-state electrolyte shows a remarkable long-term performance at high current rates (1, 2, 4, and 5C) and rate capabilities at 0.1, 0.2, 0.5, 1, 2, 4 and back to 0.1C. The battery was able to recover 77% of the initial speciﬁc capacity at 0.1C. The materials were characterized by XRD and SEM-EDX to study the crystallinity and elemental distributions, respectively.


Introduction
Lithium-ion batteries have been widely used as energy storage devices for portable electronics, electric vehicles, and power grids [1,2].However, their low energy density, poor cycle stability, flammable organic liquid electrolytes, and high cost encourage research toward finding better alternatives to avoid these problems.Lithium-sulfur batteries could be the next evolutionary step in the quest for technology beyond the traditional lithiumion batteries due to their superior properties, such as the theoretical specific capacity of the sulfur electrode of 1672 mAh g −1 [3][4][5].Despite having a high theoretical capacity, their wide usage has been ebbed by some prohibiting shortcomings such as diminished cycle life due to the shuttle effect reaction and the flammability of the organic solvent employed in the electrolyte [6,7].Many studies have been working on solving the problems mentioned above.Combining carbon materials, such as mesoporous carbon, hollow porous carbon, carbon nanotube, and graphene with sulfur were investigated to improve the sulfur utilization [8][9][10][11][12][13].Using solid state electrolytes has been gaining more and more attention to overcome the dendrite formation and shuttling effects of the polysulfides [14][15][16][17].There are four main types of solid electrolytes including gels, solid polymers, inorganic ceramics, and inorganic-organic composite electrolytes.Although inorganic ceramics possess a high ionic conductivity (~10 −3 S cm −1 at room temperature), they have poor mechanical properties, poor interfacial contact between the solid electrode and the solid electrolyte, and instability in air [18][19][20].The ionic conductivity of solid polymer electrolytes is low (~10 −6 S cm −1 at Energies 2022, 15, 7827 2 of 11 room temperature); however, they are more flexible, stretchable, and easier to be fabricated than inorganic ceramics [21].
Several studies have used different approaches to improve the ionic conductivity of polymer electrolytes.It has been concluded that minimizing the crystallinity of polymers could enhance the ionic conductivity of polymer electrolytes [22][23][24].Incorporating metal-organic frameworks (MOFs) into the matrix of polymers is a successful approach to improve the ionic conductivity of polymer electrolytes due to the nanoporous structure of MOFs.The nanoporous structure of MOFs provide ideal accommodation for absorbing Li-containing compounds or mixtures in the nanopores that converts it into lithium-ion conductors [25,26].For instance, a typical MOF material (UIO-66) was added to the poly(ethylene oxide) (PEO) solid electrolyte for lithium-ion battery [27].A nonvolatile and nonflammable Li-containing ionic liquid (Li-IL), i.e., LiTFSI in 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide [EMIM][TFSI], was added and showed an improvement in the lithium ions transport and cycling stability.In the first cycle, the specific charge capacity of the battery is 151 mAh g −1 at a rate of 0.5C [27].The specific discharge capacities of the 10th, 50th, and 100th cycles were measured and found to be 157, 152, and 144 mAh g −1 , respectively.The efforts of Han et al. studied the use of MOF as a filler for PEO electrolyte in the lithium-ion battery [28].The battery delivered an initial specific capacity of 144 mAh g −1 with a high Coulombic efficiency of 98.9%.It has been shown that incorporating a lithium-ion-intercalating material such as lithium cobalt oxide (LCO) could improve the facile transport of lithium ions in the matrix of the active material and the electrolyte [29].
In this work, the ionic conductivity of PEO will be enhanced by reducing its crystallinity and improving the lithium ions transport.To the best of our knowledge, incorporating transition metal carbides supported on mesoporous silica (SBA-15) as a filler in the solid electrolytes for lithium-sulfur batteries has not been investigated.In this study, we propose a new novel composite solid-state electrolyte, which is nickel-tungsten carbides (NiWC) over SBA-15 filled PEO, for lithium-sulfur batteries that can be operated at room temperature.This new composite electrolyte was modified by adding IL and LCO to enhance the capacity and stability of the Li-S battery.

Preparation of the Solid Electrolyte (PEO-LiTFSI-NiWC/SBA-15-IL-LCO)
PEO was dried at 50 • C, and LiTFSI was dried at 100 • C for 24 h under vacuum and stored in an Ar-filled glove box.Then, 1.0 g PEO was mixed with 0.4 g LiTFSI, dissolved in 30 mL acetonitrile, and magnetically stirred at 60 • C for 2 h.The LCO was added to the mixture with a weight ratio of 60:40.A 30 wt% of the nanostructured NiWC/SBA-15-IL was added to the mixture, and then sonicated for 10 min to get a homogeneous solution.Finally, the solution was magnetically stirred at 60 • C for 24 h.

Preparation of the Cathode (S-CoS@CNT-PEO-LiTFSI-NiWC/SBA-15-IL-LCO)
The cathode electrode slurry was prepared by melt-diffusion method of sulfur with 20%-CoS@multi-wall carbon nanotubes (CoS@MWCNT) with a weight ratio of 70 wt% to 30 wt% as reported previously [32].The mixture was mixed with Super P with a weight ratio of 72:28.PVDF and LCO were added to the mixture with a weight ratio of the mixture, PVDF, and LCO as 60, 33, and 7, respectively.The prepared PEO-LiTFSI-NiWC/SBA-15-IL was added to the mixture with a weight ratio of 40 to 60.An adequate amount of NMP was added to the mixture as a solvent to form a slurry.After grinding the slurry for 90 min, the slurry was coated on aluminum foil by the doctor blade, and then dried at 60 • C under vacuum for 6 h.

Materials Characterization
X-ray Diffraction (XRD) patterns of the materials were acquired by a Bruker D2 Phaser using Cu Kα radiation at room temperature to analyze the crystallinity of the materials at angles from 10 to 80 • .The elemental distribution on the cathode and electrolyte were characterized by the Field-Emission Scanning Electron Microscopy (JSM-7600 FE SEM).

The Assembly of the Cells
The cathode surface was directly coated by the solid polymer electrolyte slurry using the doctor blade to minimize the interfacial resistance between the cathode and the electrolyte.The cathode/electrolyte material was dried at 60 • C under vacuum for 6 h.The cathode-electrolyte material and lithium were cut with diameters of 14 mm and 12 mm, respectively.The cells were made using LIR2032 coin cells.All cells were assembled in an Ar-filled glovebox (O 2 < 0.1 ppm, H 2 O < 0.1 ppm).

Electrochemical Measurements
Galvanostatic discharge-charge tests were conducted by Maccor Model 4200 Automated Test System between a voltage range of 1.5-3.0V (vs.Li/Li + ) at room temperature.The electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) experiments were tested by using a CH Instruments CHI660D Electrochemical Workstation.The scan rate of CV is 0.1 mV/s and the frequency of EIS is from 0.1 to 1,000,000 Hz.

Materials Crystallography
Figure 1 shows the XRD patterns of the PEO, NiWC/SBA-15 (Ni/W = 1:1), and PEO/NiWC/SBA-15 (Ni/W = 1:1).The XRD pattern of the PEO has two prominent peaks at 19.1 • and 23.3 • , indicating that the PEO has high level of crystalline structure [33].The XRD pattern of the NiWC/SBA-15 (Ni/W = 1:1) shows an amorphous structure due to the existence of SBA-15.The intensities of the peaks of PEO become weaker after adding the NiWC/SBA-15 (Ni/W = 1:1), suggesting that the degree of crystallization of PEO could be lowered by the addition of NiWC/SBA-15 (Ni/W = 1:1).Therefore, the segmental motion of the polymer chains of PEO could be increased, which leads to an increase in the ionic conductivity [31].

The Electrochemical Performance
The EIS measurements shown in Figure 2 illustrate the impedance for S-CoS@CNT-PEO-LiTFSI-LCO/PEO-LiTFSI-LCO/Li metal (SPL) cell, and Figure 3 presents the impedance for both S-CoS@CNT-PEO-LiTFSI-NiWC/SBA-15-IL-LCO/PEO-LiTFSI-NiWC/SBA-15-IL-LCO/Li metal (SPN1W1L) with a ratio of Ni/W = 1:1 and SPN9W1L with a ratio of Ni/W = 9:1 cells before cycling.A significant reduction in the impedance were occurred after incorporating the NiWC/SBA-15-IL and LCO into the matrix of the electrolyte and the cathode.The impedance was lowered from 26,256 ohm to 2416 ohm and to 5734 ohm after adding the electrolyte material with Ni/W = 1:1 and with Ni/W = 9:1, respectively.This suggests that the amorphous structure of NiWC/SBA-15 provides accommodation for absorbing Li-containing compounds in the nanopores that makes it a better lithiumion conductor [25,26].Moreover, the addition of LCO promotes the movement of lithium ions in the cathode and the electrolyte [29].

The Electrochemical Performance
The EIS measurements shown in Figure 2 illustrate the impedance for S-CoS@CNT-PEO-LiTFSI-LCO/PEO-LiTFSI-LCO/Li metal (SPL) cell, and Figure 3 presents the impedance for both S-CoS@CNT-PEO-LiTFSI-NiWC/SBA-15-IL-LCO/PEO-LiTFSI-NiWC/SBA-15-IL-LCO/Li metal (SPN1W1L) with a ratio of Ni/W = 1:1 and SPN9W1L with a ratio of Ni/W = 9:1 cells before cycling.A significant reduction in the impedance were occurred after incorporating the NiWC/SBA-15-IL and LCO into the matrix of the electrolyte and the cathode.The impedance was lowered from 26,256 ohm to 2416 ohm and to 5734 ohm after adding the electrolyte material with Ni/W = 1:1 and with Ni/W = 9:1, respectively.This suggests that the amorphous structure of NiWC/SBA-15 provides accommodation for absorbing Li-containing compounds in the nanopores that makes it a better lithium-ion conductor [25,26].Moreover, the addition of LCO promotes the movement of lithium ions in the cathode and the electrolyte [29].

The Electrochemical Performance
The EIS measurements shown in Figure 2 illustrate the impedance for S-CoS@CNT-PEO-LiTFSI-LCO/PEO-LiTFSI-LCO/Li metal (SPL) cell, and Figure 3 presents the impedance for both S-CoS@CNT-PEO-LiTFSI-NiWC/SBA-15-IL-LCO/PEO-LiTFSI-NiWC/SBA-15-IL-LCO/Li metal (SPN1W1L) with a ratio of Ni/W = 1:1 and SPN9W1L with a ratio of Ni/W = 9:1 cells before cycling.A significant reduction in the impedance were occurred after incorporating the NiWC/SBA-15-IL and LCO into the matrix of the electrolyte and the cathode.The impedance was lowered from 26,256 ohm to 2416 ohm and to 5734 ohm after adding the electrolyte material with Ni/W = 1:1 and with Ni/W = 9:1, respectively.This suggests that the amorphous structure of NiWC/SBA-15 provides accommodation for absorbing Li-containing compounds in the nanopores that makes it a better lithiumion conductor [25,26].Moreover, the addition of LCO promotes the movement of lithium ions in the cathode and the electrolyte [29].Figure 4 shows the specific capacities at room temperature in a voltage window o 1.5-3.0V at a rate of C/10 for both SPN1W1L and SPN1W9L cells.The initial specific ca pacity of 1305 mAh g −1 and 478 mAh g −1 was obtained for SPN1W1L and SPN1W9L, re spectively.The specific capacity of SPN1W1L leveled off after the third cycle with a ca pacity retention of 66.7% after eight cycles.This compared favorable to the best reported finding for Li-S polymer-electrolyte-system with an initial energy capacity of around 1481 mAh g −1 , and a capacity retention of approximately 60.0% after eight cycles at a lower current rate of C/20 [29].
The SPN1W1L cell was further investigated to study the electrochemical perfor mance.Cyclic voltammograms have been recorded in the voltage range of 1.5-3.0V at a scan rate of 0.1 mV/s.Figure 5a shows peaks around 2.1 V and 2.5 V in the first scan which are attributed to the reduction of sulfur to lithium polysulfides (Li2Sn, 2 < n < 8) and oxi dation of lithium sulfides to elemental sulfur and lithium, respectively [34,35].During the second cycle of the voltammetry, and onwards (Figure 5b), there were no significan changes in the reduction and oxidation peaks.The anodic peak area is smaller than tha of the cathodic peak due to the high sweeping rate applied for the experiments, as the rate of mass transfer of Li + to S and polysulfides is higher than that of the rate of electron trans fer [36].Figure 4 shows the specific capacities at room temperature in a voltage window of 1.5-3.0V at a rate of C/10 for both SPN1W1L and SPN1W9L cells.The initial specific capacity of 1305 mAh g −1 and 478 mAh g −1 was obtained for SPN1W1L and SPN1W9L, respectively.The specific capacity of SPN1W1L leveled off after the third cycle with a capacity retention of 66.7% after eight cycles.This compared favorable to the best reported finding for Li-S polymer-electrolyte-system with an initial energy capacity of around 1481 mAh g −1 , and a capacity retention of approximately 60.0% after eight cycles at a lower current rate of C/20 [29].The SPN1W1L cell was further investigated to study the electrochemical performance.Cyclic voltammograms have been recorded in the voltage range of 1.5-3.0V at a scan rate of 0.1 mV/s.Figure 5a shows peaks around 2.1 V and 2.5 V in the first scan which are attributed to the reduction of sulfur to lithium polysulfides (Li 2 S n , 2 < n < 8) and oxidation of lithium sulfides to elemental sulfur and lithium, respectively [34,35].During the second cycle of the voltammetry, and onwards (Figure 5b), there were no significant changes in the reduction and oxidation peaks.The anodic peak area is smaller than that of the cathodic peak due to the high sweeping rate applied for the experiments, as the rate of mass transfer of Li + to S and polysulfides is higher than that of the rate of electron transfer [36].Figure 6 shows the discharge capacities of the SPN1W1L cell were measured at different current rates (1, 2, 4, and 5C) at room temperature for over 100 cycles.High initial discharge capacities of 526, 485, 190, and 174 mAh g −1 were obtained at 1, 2, 4, and 5C, respectively.However, the specific capacities started decreasing due to several reasons.Firstly, the capacity fading can be partly attributed to the increase bulk resistances as indicated by the second semicircles in the EIS spectra (Figure 7) after 100 cycles at 2, 4, and 5C [37].Moreover, the bulk resistances were increased to 8.3 k, 11.2 k, and 12.1 k after running the batteries at 2, 4, and 5C, respectively.Secondly, the SEM and EDX mapping (Figure 8) shows the migration of W from the solid electrolyte toward the cathode side that might contributing to the increase in electrolyte resistance.Figure 8a shows that the S, Ni, and W were very well dispersed over the cathode and the electrolyte before cycling.However, the distribution of Ni and W were largely disturbed after 100 cycles at 2C.The migration of Ni and W particles lowers the content of the transition metal fillers in the electrolyte which causes lowering the ionic conductivity of the electrolyte; therefore, decreasing the specific capacities of the batteries.Finally, the volume expansion occurs during the discharge of the cathode where sulfur is converted to lithium disulfide could disturb the solid electrolyte structure and have an effect on lowering the specific capacity of the batteries [29].The average coulombic efficiency is about 92%, while the capacity Figure 6 shows the discharge capacities of the SPN1W1L cell were measured at different current rates (1, 2, 4, and 5C) at room temperature for over 100 cycles.High initial discharge capacities of 526, 485, 190, and 174 mAh g −1 were obtained at 1, 2, 4, and 5C, respectively.However, the specific capacities started decreasing due to several reasons.Firstly, the capacity fading can be partly attributed to the increase bulk resistances as indicated by the second semicircles in the EIS spectra (Figure 7) after 100 cycles at 2, 4, and 5C [37].Moreover, the bulk resistances were increased to 8.3 k, 11.2 k, and 12.1 k after running the batteries at 2, 4, and 5C, respectively.Secondly, the SEM and EDX mapping (Figure 8) shows the migration of W from the solid electrolyte toward the cathode side that might contributing to the increase in electrolyte resistance.Figure 8a shows that the S, Ni, and W were very well dispersed over the cathode and the electrolyte before cycling.However, the distribution of Ni and W were largely disturbed after 100 cycles at 2C.The migration of Ni and W particles lowers the content of the transition metal fillers in the electrolyte which causes lowering the ionic conductivity of the electrolyte; therefore, decreasing the specific capacities of the batteries.Finally, the volume expansion occurs during the discharge of the cathode where sulfur is converted to lithium disulfide could disturb the solid electrolyte structure and have an effect on lowering the specific capacity of the batteries [29].The average coulombic efficiency is about 92%, while the capacity retentions after 100 charge-discharge cycles were 34, 5, 4, and 4% at 1, 2, 4, and 5C, respectively.The rate capability of SPN1W1L was investigated by cycling at charge-discharge rates varying from 0.1C to 4C with cutoff voltages of 1.5 and 3.0 V (Figure 9).Although the capacity dropped significantly at a high C-rate of 4C, 77% of the 0.1C capacity was recovered when lowering the rate again to 0.1C.These results suggest that the sulfur utilization was impeded at high C-rate; therefore, the sulfur electrode polarization is an important issue to address [38].The recovering of 77% of the capacity after lowering the discharge rate highlights the good performance of the all-solid-state Li-S battery SPN1W1L.In this study, it has been demonstrated that the addition of transition metal carbides supported on mesoporous silica (NiWC/SBA-15) reduces the crystallinity of the PEO due to the amorphous structure of the SBA-15, and increases the ionic conductivity of the solid electrolyte due to the high conductivity of the metal carbide phase [39].Moreover, this study indicated that the existence of LCO and IL are essential to provide a facile transport of lithium ions in the matrix of the electrolyte [29] and to increase the ionic conductivity of the electrolyte [31], respectively.Compared to the previous studies, Elizalde-Segovia et al. [29] utilized the LCO as a second layer in the solid electrolyte without adding the The rate capability of SPN1W1L was investigated by cycling at charge-discharge rates varying from 0.1C to 4C with cutoff voltages of 1.5 and 3.0 V (Figure 9).Although the capacity dropped significantly at a high C-rate of 4C, 77% of the 0.1C capacity was recovered when lowering the rate again to 0.1C.These results suggest that the sulfur utilization was impeded at high C-rate; therefore, the sulfur electrode polarization is an important issue to address [38].The recovering of 77% of the capacity after lowering the discharge rate highlights the good performance of the all-solid-state Li-S battery SPN1W1L.The rate capability of SPN1W1L was investigated by cycling at charge-discharge rates varying from 0.1C to 4C with cutoff voltages of 1.5 and 3.0 V (Figure 9).Although the capacity dropped significantly at a high C-rate of 4C, 77% of the 0.1C capacity wa recovered when lowering the rate again to 0.1C.These results suggest that the sulfur uti lization was impeded at high C-rate; therefore, the sulfur electrode polarization is an im portant issue to address [38].The recovering of 77% of the capacity after lowering the discharge rate highlights the good performance of the all-solid-state Li-S battery SPN1W1L.In this study, it has been demonstrated that the addition of transition metal carbide supported on mesoporous silica (NiWC/SBA-15) reduces the crystallinity of the PEO due to the amorphous structure of the SBA-15, and increases the ionic conductivity of the solid electrolyte due to the high conductivity of the metal carbide phase [39].Moreover, thi study indicated that the existence of LCO and IL are essential to provide a facile transpor of lithium ions in the matrix of the electrolyte [29] and to increase the ionic conductivity of the electrolyte [31], respectively.Compared to the previous studies, Elizalde-Segovia e al. [29] utilized the LCO as a second layer in the solid electrolyte without adding the In this study, it has been demonstrated that the addition of transition metal carbides supported on mesoporous silica (NiWC/SBA-15) reduces the crystallinity of the PEO due to the amorphous structure of the SBA-15, and increases the ionic conductivity of the solid electrolyte due to the high conductivity of the metal carbide phase [39].Moreover, this study indicated that the existence of LCO and IL are essential to provide a facile transport of lithium ions in the matrix of the electrolyte [29] and to increase the ionic conductivity of the electrolyte [31], respectively.Compared to the previous studies, Elizalde-Segovia et al. [29] utilized the LCO as a second layer in the solid electrolyte without adding the transition Energies 2022, 15, 7827 9 of 11 metal carbides in Li-S battery.The Li-S cells delivered an initial specific capacity of around 1481 mAh g −1 , and a capacity retention of approximately 60.0% after eight cycles, but at a lower current rate of C/20 [29]; however, the long-term performance of the battery was not investigated and only eight cycles were reported.This study shows that the SPN1W1L not only can achieve a high initial specific capacity of 1305 mAh g −1 and better capacity retention of 66.7% after eight cycles at a higher current rate of C/10, but also shows a longer-term performance with higher current rates, and good rate capability.

Conclusions
This is the first study of transition metal carbides supported on mesoporous silica (NiWC/SBA-15) as a filler in the solid composite polymer electrolyte for Li-S batteries.The addition of NiWC/SBA-15 reduced the resistance of the all-solid-state Li-S battery due to the existence of amorphous structure of SBA-15 and the high conductivity of the NiW carbide phase.The results were a breakthrough in terms of initial specific conductivity, rate capability, and long-term performance.The initial specific capacity and capacity retention of SPN1W1L were 1305 mAh g −1 and 66.7% after eight cycles, respectively.Moreover, it shows great potential to perform at high rates (1, 2, 4, and 5C) and shows the ability to accommodate the changing in current rates in the order of 0.1, 0.2, 0.5, 1, 2, 4, then 0.1C, as well as regaining 77% of the initial specific capacity.Using transition metal carbides supported on mesoporous silica is a novel and promising approach.Further investigations, such as stabilizing the TMC within the PEO matrix, and the effect of mono-and tri-transition metal carbides on the performance of Li-S batteries should be made.