Electrochemical performance study of nano-Fe3O4/C modified separator for lithium-sulfur batteries

In order to solve the problems of low cathode conductivity, shuttle effect, and poor electrochemical performance of Li-S batteries, we designed a porous carbon/nano-sized ferric oxide (Fe3O4) composite modified membrane. This improved both conductivity and adsorption capacity of lithium polysulfides (LiPSs) prepared with mesopores and high specific surface area C-Fe3O4@PE. The membrane can bind polysulfide via chemical and physical adsorption. Owing to these advantages, a Li-S battery based on C-Fe3O4@PE and with a separator exhibited excellent rate performance (965.9 mA h g–1 at 0.2 C and 437.5 mA h g–1 at 5 C), as well as a long cycle life (416.6 mA h g–1 after 300 cycles at 0.5 C, with a capacity retention rate of 50.5%).


Introduction
Currently, the insufficient energy density of Li-ion cells poses a significant challenge to the advancement and widespread adoption of portable electronics and electric vehicles.Therefore, the development of innovative battery systems with superior specific energy density becomes imperative.Li-S cells, with a theoretical energy density of 2,600 Wh kg -1 , have emerged as a promising candidate due to their cost-effectiveness, environmental friendliness, and ample storage capacity [1][2][3][4][5].However, there are substantial challenges that hinder the practical realization of Li-S batteries.Short lifespan, primarily attributed to the dissolution of LiPSs and the significant volume changes during charge and discharge, is a major concern.Additionally, the current sulfur loading is insufficient for practical energy storage applications.Furthermore, the poor electronic and ionic conductivity of sulfur and the discharge product, Li 2 S, contribute to sluggish reaction kinetics and substantial polarization, further compromising battery performance [6][7][8][9][10].
To address these challenges, researchers have devoted significant efforts to finding solutions.Carbonaceous materials have been widely investigated as hosts for sulfur in Li-S cells due to their ability to improve conductivity, trap soluble LiPSs, and mitigate volume changes in the sulfur cathode [11][12][13][14][15][16][17].However, conventional carbon materials lack polarity, making it difficult to effectively immobilize polar polysulfides.In contrast, polar transition metal oxides or metal sulfides exhibit strong chemisorption capabilities towards polysulfides and effectively suppress the shuttle effect.Nevertheless, the low electrical conductivity of most metal oxides limits the multiplicative performance of the cell.Additionally, the high cost of synthesizing the primary materials, the complexity of multi-step processes, and the stringent conditions involved pose challenges for achieving commercial viability.
One of the effective and straightforward approaches to mitigate the shuttle effect in Li-S batteries is to modify the separator by introducing a conductive layer between the separator and the cathode.This strategy has demonstrated success in impeding the shuttle of LiPSs and improving the utilization of the active material [18].In most studies, carbon materials are utilized as a coating to modify the separator, physically impeding the diffusion of soluble LiPSs to the negative electrode.However, the interaction between non-polar carbon and polar LiPSs is weak, resulting in limited binding effectiveness and poor electrochemical stability.To address this limitation, polar materials that can chemically interact with LiPSs have been investigated to enhance the overall electrochemical performance of Li-S cells.Iron tetroxide (Fe 3 O 4 ), with its low cost, good electrical conductivity (5 × 10 4 S m −1 ), and strong chemical interactions with polysulfides, has been identified as a promising polar material [19,20].Nevertheless, the large size of Fe 3 O 4 hinders its adsorption of LiPSs, highlighting the need to reduce its size to maximize the utilization of the active material.
In this study, we have developed a modified separator for Li-S batteries by incorporating Fe 3 O 4 nanoparticles and porous carbon powder.By combining the polar properties of Fe 3 O 4 with the physical interactions provided by porous carbon, this approach effectively blocks polysulfides.The high specific surface area of porous carbon also holds the potential to significantly enhance electrode conductivity.Benefitting from these advantages, Li-S batteries utilizing C-Fe 3 O 4 @PE modified separators exhibit remarkable electrochemical properties.Specifically, they achieve a specific capacity of 965.9 mAh g -1 at 0.2 C, a multiplicative performance of 437.5 mAh g -1 at 5 C, and maintain cycling stability with a capacity of 416.6 mAh g -1 at 0.5 C after 300 cycles (with a capacity retention rate of 50.5%).

Preparation of materials and cell assemblies
2.1.Materials Nano Iron (II, III) oxide (99.0%metals basis), polyvinylidene difluoride (PVDF), and N-Methylpyrrolidone (NMP) were purchased from Aladdin.All the reagents were of analytical grade and were used without any additional purification.The size of the spherical nano Iron (II, III) oxide particles is 20 nm.The polyethylene (PE) membrane was also obtained from Aladdin and has a pore size of 0.1 μm.

Preparation of C/S composites
In the preparation procedure, the porous carbon and sublimated sulfur powders were weighed at a ratio of 1:4.The weighed powders were then subjected to mechanical ball milling in a ball mill for a duration of 12 h.Subsequently, the powder mixture was transferred to a reactor liner and dried in a vacuum drying oven for 6 h.The reactor liner, along with the dried powder, was promptly transferred to a glove box and assembled in a sealed manner.Finally, the assembled reactor was placed in a blast dryer and heated at 155 °C for 20 h.This process yielded the C/S composite.

C-Fe 3 O 4 @PE separator preparation
To prepare C-Fe 3 O 4 @PE separators, slurries were formulated by combining C and Fe 3 O 4 powders with PVDF in NMP, following a mass ratio of 11:4:5 (C: Fe 3 O 4 : PVDF).After blending and 5 h of stirring in a ball grinder, the resulting mixture was uniformly coated onto a commercially available PE separator.The coated separator was then subjected to vacuum drying at 60 °C for 12 h.Subsequently, the resulting membrane, referred to as C-Fe 3 O 4 @PE, was cut into circles with a diameter of 16 mm.The loading mass of the coating on the cell separator was controlled at 0.3 mg cm −2 .For comparison, a carbon-coated (C@PE) separator was prepared using the same method, with a C:PVDF ratio of 7:3.

Preparation of electrode sheets
The C/S composite, C, and PVDF were weighed at a ratio of 8:1:1 and subjected to mechanical ball milling at 400 r min −1 for 6 h in a ball milling tank.Throughout the ball milling process, NMP solution was continuously added to the slurry to ensure homogeneity.Subsequently, the slurry was uniformly coated onto the surface of an aluminum foil, and the coated foil was placed in a vacuum drying oven for 12 h to complete the drying process.Finally, the dried foil was sliced into circles with a diameter of 13 mm, which served as positive electrode sheets (referred to as electrode S).The electrode S had a loading of 1 mg cm -2 ., respectively.These peaks are consistent with the PDF card for Fe 3 O 4 (PDF No. .However, in the C-Fe 3 O 4 @PE separator, two intense diffraction peaks at 21.6°and 24.0°are observed in the ultra-high PE region (PDF No. 53-1859).As a result, the Fe 3 O 4 peak is weakened, and only a few faint peak features are visible, similar to the C@PE separator.The broad peak near 25°i n the C@PE separator corresponds to the diffraction peak of C. To further analyze the porous structure of C-Fe 3 O 4 @PE and C@PE separators, we investigated their specific surface area and pore size distribution using type IV nitrogen adsorption isotherms (figures 2(a)-(b)).These isotherms revealed the presence of mesopores.It is worth noting that the inclusion of porous carbon significantly increased the specific surface area of both C@PE and C-Fe 3 O 4 @PE separators, as detailed in table S1.The higher specific surface area and the presence of suitable mesopores in the C-Fe 3 O 4 @PE separator facilitated the infiltration of electrolytes, ensuring excellent ion conductivity for the separator.

Experimental results and discussion
To directly examine the morphology of the newly developed C-Fe 3 O 4 @PE separator, SEM is employed.Figure 3

Electrochemical performance analysis of modified lithium-sulfur battery separators
To provide a comprehensive evaluation of the outstanding performance of C-Fe 3 O 4 @PE in Li-S batteries, we assembled a 2025 type button cell and conducted corresponding electrochemical performance tests.Figure 5(a) presents the charge/discharge curves of Li-S cells utilizing two distinct separators, namely C-Fe 3 O 4 @PE and C@PE, at a magnification of 0.2 C. It was observed that Li-S cells with the C-Fe 3 O 4 @PE separator exhibited a higher discharge capacity (965.9 mAh g -1 ), surpassing that of Li-S cells employing C@PE separators (802.4 mAh g -1 ).The Li-S batteries with two different separators exhibited a higher discharge capacity compared to Li-S batteries with PE separators (713.0 mAh g −1 ) (figure S1).Table 1 summarises the improved electrochemical performance of Li-S batteries with C-Fe 3 O 4 coating and other types of coatings reported in the literature.These results indicate that the cells coated with C-Fe 3 O 4 in this work have excellent electrochemical performance.Additionally, the polarization of cells with the C-Fe 3 O 4 @PE separator is lower compared to that of cells with the C@PE separator, indicating enhanced redox reaction kinetics at the electrode-electrolyte interface.Batteries with C-Fe 3 O 4 coated diaphragms offers the longest discharge plateau at potentials around 2.1 V,  To further investigate the electrochemical performance of Li-S cells, CV tests were conducted on cells utilizing C-Fe 3 O 4 @PE and C@PE separators.Figures 5(c)-(d) present five CV scans of the cell using a C-Fe 3 O 4 @PE separator with a scan rate of 0.1 mV s -1 across a voltage range of 1.6-2.8V.The results reveal a significant degree of overlap between the oxidation and reduction peaks, indicating a high level of reversibility.Moreover, the superimposition of the CV curves demonstrates pronounced reversibility.Figure 5(c) display two distinct cathodic peaks, corresponding to the conversion of sulfur to long-chain LiPSs (Li 2 S x , where 4 x 8) and subsequently to short-chain LiPSs.The typical anodic peaks of Li 2 S can be attributed to the oxidation of Li 2 S to LiPSs.Comparing the cathodic potentials, it is evident that the positive potential of the C-Fe 3 O 4 @PE battery electrode (2.02 V) exceeds that of other C@PE battery electrodes (2.00 V), suggesting a lower degree of electrochemical polarization during cycling [25].To validate the facilitating effect of the C-Fe 3 O 4 layer on the cell's reaction kinetics, we analyzed the charge transfer resistance (Rct) at the electrode/electrolyte interface and the electrolyte resistance (Rs) using electrochemical impedance spectroscopy (EIS) [26][27][28].the Nyquist plots of cells employing C@PE and C-Fe 3 O 4 @PE diaphragms, respectively.The cell employing C-Fe 3 O 4 @PE diaphragm exhibited a significantly reduced charge transfer resistance (39.5 Ω) compared to the cell using an uncoated PE diaphragm (figure S2c, 117.5 Ω).This reduction can be attributed to the conductive layer on the separator, which increases the conducting surface, accelerates the rate of electron transfer, and lowers the Rct [25].Furthermore, for C-Fe 3 O 4 @PE diaphragm batteries, the incorporation of the C-Fe 3 O 4 layer not only enhances the overall electrical conductivity of the battery but also promotes stronger chemisorption of polysulfides in the electrolyte, improving the reactivity of the active substances.This accelerates the diffusion of  polysulfides into the electrolyte, resulting in an increased Rs value for the battery.impeding the diffusion of polysulfide anions.The nano Iron (II, III) oxide coating provides a physical shield against polysulfide shuttling by its mesoporous structure and chemical adsorption of polysulfides by Fe 3 O 4 [29,30].

Conclusion
In conclusion, we have successfully synthesized novel C-Fe 3 O 4 @PE functional separators by incorporating Fe 3 O 4 nanoparticles into C-modified PE separators.These separators possess distinctive structural and chemical properties that contribute to the enhancement of electrochemical performance in Li-S batteries.The resulting C-Fe 3 O 4 @PE separator, characterized by mesopores and a high specific surface area, effectively immobilizes polysulfides through both chemical and physical adsorption mechanisms.When employed in Li-S cells, the C-Fe 3 O 4 @PE separators exhibit outstanding performance at different charge-discharge rates, achieving a capacity of 965.9 mAh g -1 at 0.2 C and 437.5 mAh g -1 at 5 C.Moreover, these cells demonstrate prolonged cycle life, maintaining a capacity of 416.6 mAh g -1 at 0.5 C after 300 cycles, with a capacity retention of 50.5%.These findings highlight the potential application of combining C materials with polar components to develop multifunctional modified separators for advanced energy storage devices.

3. 1 .
Structural analysis of modified lithium-sulfur battery separators Figure 1(a) displays the XRD diffraction patterns of Fe 3 O 4 obtained from the C-Fe 3 O 4 @PE separators.The diffraction peaks at 31.2°, 36.8°,44.7°, 59.3°, and 65.1°correspond to the crystal planes (220), (311), (400), (511), and (440) of the Fe 3 O 4 phase (PDF No. (a) presents an SEM image of the cross-section of the C-Fe 3 O 4 @PE separator, revealing a surface coverage of Fe 3 O 4 with a coating thickness of approximately 5 μm (figure 3(b)).To ensure a fair comparison, the thickness of the C-modified separator coating was also controlled at 5 μm (figure 4(b)).Furthermore, the elemental mapping analysis of iron (Fe), carbon (C), and oxygen (O) demonstrates the uniform distribution of Fe 3 O 4 on the separator surface (figures 3(c)-(e)).

Figure 1 .
Figure 1.(a) XRD images of Fe 3 O 4 and C-Fe 3 O 4 @PE separator; (b) XRD image of C and C@PE separator.

Figure 2 .
Figure 2. (a) Pore size distribution of coat materials for both separators; (b) Adsorption/desorption curves of coat materials for both separators.

Figure 3 .
Figure 3. (a) Planar SEM image of C-Fe 3 O 4 @PE separator; (b) Cross-sectional SEM image of C-Fe 3 O 4 @PE separator; (c-e) Mapping of Fe, O and C elements in C-Fe 3 O 4 @PE separator.

Figure 4 .
Figure 4. (a) SEM image of a C@PE separator in plane; (b) SEM image of a C@PE separator in cross-section.

Figure 5 .
Figure 5. (a) Initial charging/discharging profiles of batteries using C-Fe 3 O 4 @PE and C@PE separators at a 2 C rate.(b) Charging/ Discharging profiles of batteries with C-Fe 3 O 4 @PE separators at different rates: 0.2, 0.5, 1, 2, and 5 C. (c) CV profiles of batteries with PE, C-Fe 3 O 4 @PE, and C@PE separators at a scan rate of 0.1 mV s -1 .(d) CV profiles of batteries with C-Fe 3 O 4 @PE separators over 5 cycles.(e) Nyquist plots and equivalent circuits of Li-S batteries with C-Fe 3 O 4 @PE and C@PE separators.(f) Coulombic efficiency and cycling stability profiles over 500 cycles at a 0.5 C rate.

Figure 5 (
f) displays the results of prolonged cycling tests conducted on both batteries.The capacity of the battery utilizing the C-Fe 3 O 4 @PE separator remained at 416.6 mAh g -1 under a 0.5 C rate after 300 cycles, exhibiting capacity retention of 50.5%.Notably, this capacity retention exceeds that of the battery with the C@PE separator by 36.5%.These results suggest that the incorporation of Fe 3 O 4 improves the binding effect of LiPSs while inhibiting the shuttle effect.To investigate the molecular interactions between the C-Fe 3 O 4 layer and polysulfides, we prepared a 2 mM Li 2 S 6 solution for visualization and detection.The adsorption experiments were characterized using UV-vis spectroscopy (figure6).After 24 h of static incubation, the initially yellow colored Li 2 S 6 solution transformed into a nearly transparent color due to adsorption by C-Fe 3 O 4 (figure6).Furthermore, the UV-visible absorption spectrum of the pristine Li 2 S 6 solution at around 260 nm shows the greatest decrease in intensity at approximately 260 nm after adsorption by C-Fe 3 O 4 , signifying that the supernatant exhibited the lowest concentration of LiPSs and the highest adsorption capacity for LiPSs.We also conducted a polysulfide diffusion barrier experiment, as depicted in figure S2 in the Supporting Information.In the experimental group employing the C@PE diaphragm, the polysulfides completely diffused to the other side of the diaphragm within 12 h.Conversely, in the experimental group employing the C-Fe 3 O 4 @PE diaphragm, the polysulfides exhibited only slight diffusion to the other side of the diaphragm within the same 12-h period.Considering the analysis above, the potential mechanism by which the C-Fe 3 O 4 layer enhances electrocatalytic activity is depicted in figure7.The C-Fe 3 O 4 @PE separator significantly enhances the cyclic stability of Li-S batteries by effectively

Figure 6 .
Figure 6.UV-vis absorption spectra of different solutions after sufficient immersion in Li 2 S 6 solution for 24 h.Inset: Optical photograph of Li 2 S 6 adsorption experiment.

Figure 7 .
Figure 7. Schematic illustration for the Li-S battery with a C-Fe 3 O 4 @PE separator.

Table 1 .
Electrochemical properties of various functional separators in Li-S batteries.