Fe 2 O 3 nano particles embedded Fe 2 O 3 /BP2000 composite for Li-S battery

Shuttle effect of lithium polysul�des in lithium sulfur batteries greatly in�uenced their commercialization. Therefore, it is urgent to develop a cheep and effective way to alleviate the shuttle effect. Fe is an active transition element which has good catalytic ability, in this work, a simple wet impregnation method was used to make Fe ions in�ltrate into the pores of BP2000 (a kind of commercial conductive carbon), then it was calcined in N 2 atmosphere to get a Fe 2 O 3 /BP2000 composite and used as a separator modi�cation layer. The Fe 2 O 3 nano particles are decorated in the pores of BP2000 which greatly enhanced the absorption ability on lithium polysul�des, additionally it also has excellent catalytic effect on lithium polysul�des, thus the Fe 2 O 3 /BP2000 layer can be served as a secondary collector to re-engage the polysul�des in the cathode reaction. In this way, the lithium sulfur batteries use the Fe 2 O 3 /BP2000 modi�ed separator show impressive electrochemical performances.


Introduction
The rapid development of electronic devices and electrical vehicles stimulates the demands for high performance secondary batteries.In recent years, Li-S battery has attracted much notice based on the high theoretical capacity of 1675 mAh g − 1 for sulfur cathode.What is more, the advantages of natural abundance, cheep price, low pollution make Li − S battery become one of the most promising secondary batteries [1][2][3].Despite these advantages, the practical use of Li-S battery still faces several challenges.First of all, the insulating nature of sulfur induces low active material utilization.Secondly, the volume difference between sulfur and end product Li 2 S can cause the positive electrode to crack.Thirdly, the migration of soluble lithium polysul des between cathode and anode which is called "shuttle effect" can lead to several obstacles such as low Coulombic e ciency, poor durability, Lithium dendrite deposition in the anode, etc. [4][5][6].
Thus, a variety of approaches to address the above obstacles are investigated.For example, various carbon based materials and other materials have been reported to act as the sulfur host to enhance the conductivity of sulfur, con ne the soluble lithium polysul des and alleviate the volume change.Electrolytes are also optimized by changing the solvents, salts, and additives to decrease the solubility of lithium polysul des or slow down the migration speed of lithium polysul des.Furthermore, various strategies have been conducted to prevent the lithium dendrite deposition on the Li anode so as to improve its interfacial and structural stability [7][8][9][10].Apart from these optimizing strategies, separator modi cations in recent researches have also been demonstrated a promising method to con ne large amount of lithium polysul des in the cathode region.For instance, different carbon materials have been proved to be an excellent modi cation layer for Li-S battery separator, such as microporous carbon, graphene, and carbon nanotubes, etc. [11][12][13].Furthermore, different kinds of polar materials such as metal oxides, metal sul des etc. have also been demonstrated good modi cation materials because of their chemical interactions with lithium polysul des [14][15][16][17][18][19][20].
In this work, we deliberately combined a commercial conductive carbon named BP2000 with Fe 2 O 3 via a simple wet impregnation method followed by a high temperature calcination.The Fe 2 O 3 /BP2000 composite is used as a separator coating layer in Li-S battery.The BP2000 constructs excellent conductive network to facilitate the charge transfers inside Li-S battery.Additionally, its rich porous structure is bene cial to adsorb a large amount of polysul des.Furthermore, the Fe 2 O 3 nano particles embedded in the pores of BP2000 not only have strong chemical interaction with lithium polysul des, but also catalyze the chemical reactions during cycling.Bene t from its adsorption and catalytic ability, the loss of the active materials in the cathode can be greatly alleviated and the redox kinetics can be also accelerated, therefore the cycle and rate performance of Li-S battery are largely improved.

Experimental section
2.1 Synthesis of Fe 2 O 3 /BP2000 composite Fe 2 O 3 /BP2000 composite was prepared by a simple impregnation method followed by a high temperature calcination.In a typical preparation process, 1.5g BP2000 was dispersed homogeneously by a mixture of 120 mL ultra pure water and 30 mL absolute ethyl alcohol.At the same time 0.5695 g Fe(NO 3 ) 3 •9H 2 O was dissolved completely in 10 mL ultra pure water.Then the Fe(NO 3 ) 3 •9H 2 O solution was added dropwisely into the BP2000 suspension, in this procedure Fe 3+ ion can be absorbed by the pores of BP2000.Afterwords, the BP2000 suspension was centrifugated and dried in an air blast drying oven.At last the dried powder was put into a tube furnace and calcined at 900°C for 2 h to obtain Fe 2 O 3 /BP2000 composite.

Fabrication of the Fe 2 O 3 /BP2000 modi ed separator
The Fe 2 O 3 /BP2000 modi ed separator was prepared by a simple slurry coating method.Typically, 0.08g Fe 2 O 3 /BP2000, 0.01g super P and 0.01g PVDF were homogeneously dispersed in 4 mL NMP by vigorously stirring.The mixture was then coated onto the PE separator by a doctor blade.After the coating procedure, the Fe 2 O 3 /BP2000 coated PE separator was dried in an air blast drying oven and at last cut into small discs.For comparison purpose, BP2000 coated PE separator was also fabricated by the same procedures.The photo of the Fe 2 O 3 /BP2000 coated PE separator is shown in Figure S1, and the cross section SEM demonstrates that the coating layer is approximately 10 µm.

Fabrication of BP2000/S cathode electrode
In a typical fabrication procedure, 1 g sublimed sulfur and 4 g BP2000 are mixed and dispersed in 30 mL absolute ethyl alcohol.Then the mixture was put into a 50 mL mill pot and ball milled at 900 r/min for 3 h.Afterwards, the sulfur and BP2000 mixture was dried and put into a vacuum oven and heated at 155°C for 10 h to obtain BP2000/S composite.To fabricate BP2000/S cathode electrode, 0.8 g BP2000/S composite, 0.1 g super P and 0.1 g PVDF were dispersed in 4 g NMP and stirred in a defoaming mixer at 2500 rpm for 3 min.The slurry was then spread onto Al foil and dried at 80°C for 10 h in a vacuum oven, at last the dried Al foil was cut into circle discs at wait for use, the sulfur area density on the cathode electrode was 3 mg cm − 2 .

Materials characterization
The morphology and nano structure was characterized by eld-emission scanning electron microscope (FESEM, NOVA Nano SEM 430) and transmission electron microscope (TEM, Tecnai G2 F20 S-TWIN).Crystal structure characterization were conducted by a X-ray diffractometer (XRD, Rigaku D/Max 2500).
The porous structure and speci c surface area was determined by a surface area analyzer.

Results and discussions
Figure 1 shows the XRD patterns of BP2000 and Fe 2 O 3 /BP2000 composite.The XRD pattern of BP2000 shows two broad diffraction peaks at around 26° and 45°, refers to the (002) crystal plane of carbon.Furthermore, the XRD pattern of Fe 2 O 3 /BP2000 composite show obvious diffraction peaks at 18.3°, 30.2°, 35.6°, 37.2°, 43.3°, 53.7°, 57.3°, 62.9°, 71.3°, 74.5° and 77.4°, which match with the crystal faces of (111), (220), (311), ( 222), (400), (422), (511), (440), (620), ( 533) and (631) respectively, according to the PDF card JD39-1436 of Fe 2 O 3 , demonstrating the successful preparation of Fe 2 O 3 /BP2000 composite [21].We use N 2 adsorption -desorption experiment to estimate the BET speci c surface area and pore size distribution of BP2000 and Fe 2 O 3 /BP2000 composite.Figure 3(a) shows the N 2 adsorption-desorption isotherms of BP2000 and Fe 2 O 3 /BP2000 composite, both of the two isotherms display a type IV hysterrisis curve, demonstrating that BP2000 and Fe 2 O 3 /BP2000 composite are composed of abundant micro and meso pores, which will be bene t for the adsorption for the lithium polysul des.According to the BET calculation, the speci c surface area of BP2000 and Fe 2 O 3 /BP2000 composite are 1229 and 1057 m 2 g − 1 .The speci c surface area of BP2000 decrease a little after the combination of Fe 2 O 3 , indicating that Fe 2 O 3 occupied some of the pores.Figure 3(b) depicts the pore structures of BP2000 and Fe 2 O 3 /BP2000 composite, both of the two materials are composed of pores form 2-10 nm, and most of the pore diameter are around 3.8 nm, the pore volumes of BP2000 and Fe 2 O 3 /BP2000 composite are 2.8 and 2.54 m 3 g − 1 , the decrease of pore volume also demonstrates that Fe 2 O 3 particles are con ned by the pores of BP2000.Additionally, the relatively small pore diameter can effectively absorb the soluble lithium polysul des [22][23].
In order to directly demonstrate the absorption capability of BP2000 and Fe 2 O 3 /BP2000 composite, a L 2 S 6 solution absorption experiment was carried out.A shown in Fig. 4(a), BP2000 and Fe 2 O 3 /BP2000 were put into Li 2 S 6 solution and held for 8 h.The photo of Li 2 S 6 solution displays a dark yellow color, while the Li 2 S 6 -BP2000 solution displays a light yellow color demonstrating that Li 2 S 6 can be absorbed by BP2000.Furthermore, the Li 2 S 6 -Fe 2 O 3 /BP2000 solution after 8 h absorption displays a transparent color which demonstrates that Fe 2 O 3 /BP2000 has an excellent absorption capability on lithium polysul des.The Li 2 S 6 solution after absorption experiment was tested by UV spectroscopy, as shown in Fig. 4(b), the Li 2 S 6 UV-Vis absorption spectrum of Li 2 S 6 -Fe 2 O 3 /BP2000 solution after 8 h absorption displays the lowest absorption peak, indicating its Li 2 S 6 concentration is also the lowest which is mostly absorbed by Fe 2 O 3 /BP2000 composite [24][25].
The advantages of Fe 2 O 3 /BP2000 modi ed separator was further studied by electrochemical performances.As shown in Fig. 5(a), CV pro les of the three different batteries show apparently two reduction peaks at around 2.3 and 2.0 V and one oxidation peak at at around 2.4 V.The reduction peak at 2.3 V represents the reduction process of sulfur to soluble lithium polysul des and the reduction peak at 2.0 V represents the reduction process of soluble lithium polysul des to end product Li 2 S 2 /Li 2 S. The broad oxidation peak at around 2.4 V represents the gradual oxidation process of Li 2 S 2 /Li 2 S to sulfur.At the same time, the Fe 2 O 3 /BP2000 modi ed separator battery has the largest current density, indicating its capacity is also the largest.Additionally, the second reduction peak voltage of Fe 2 O 3 /BP2000 sample is a little bit higher than the BP2000 and PE separator sample, demonstrating it has a better reaction kinetics.
In order to further demonstrate the catalytic effect of Fe 2 O 3 /BP2000 composite, a symmetric cell experiment was carried out.The assemble process of symmetric cell and its structure diagram is given in the support information.CV measurement results of symmetric cells are shown in Fig. 5(b), as displayed in the gure, BP2000 and Fe 2 O 3 /BP2000 symmetric cells with Li 2 S 6 electrolyte show obvious current peaks, while the current of Fe 2 O 3 /BP2000 symmetric cell without Li 2 S 6 electrolyte is nearly zero.
Additionally, the current peak of Fe 2 O 3 /BP2000 symmetric cell is much larger than that of the BP2000 symmetric cell demonstrating the catalytic effect is mainly caused by the combination of Fe 2 O 3 nano particles [26].
Figure 6(a) depicts the discharge/charge pro les of the three samples at 0.1 C, all the batteries show two typical discharge platforms and one charge platform which is consistent with the reduction and oxidation peaks of CV curves.Additionally, the potential gaps of BP2000 and Fe 2 O 3 /BP2000 separator batteries is apparently smaller which indicates the remarkably improved electrochemical kinetics.Long cycle performances were performed to evaluate the discharge/charge capacities and cycle stability of BP2000 and Fe 2 O 3 /BP2000 modi ed separator.performance for more than 500 cycles, furthermore the Fe 2 O 3 /BP2000 displays a much higher discharge capacity (500 mAh g − 1 ) at the 500 cycle than that of the BP2000 sample (320mAh g − 1 ).The higher discharge capacity of Fe 2 O 3 /BP2000 battery is mainly caused by the combination of Fe 2 O 3 nano particles which can improve the absorption capability of lithium polysul des.
The rate performances of the PE, BP2000 and Fe 2 O 3 /BP2000 samples are shown in Fig. 6(c).The discharge capacities of the Fe 2 O 3 /BP2000 separator battery at different current rates are much larger than that of the BP2000 separator battery and PE separator battery.Furthermore, the Fe 2 O 3 /BP2000 separator battery displays a highly reversible capacity of 1020 mAh g − 1 at 1 C and gradually recovers to 1250 mAh g − 1 as the current was set back to 0.1 C, indicating its fast redox reaction speed.
EIS curves of different separator batteries were collected before cycle.As displayed in Fig. 6(d), the inset in the X axis are approximately the same demonstrating that the electrolyte environment are approximately the same.Furthermore, the diameter of the semicircle which represents the charge transfer resistance clearly indicates that the use of BP2000 and Fe 2 O 3 /BP2000 separator can apparently decrease the internal resistance of the Li-S battery.This can be attributed to the low conductivity of BP2000.Additionally, the oblique line of Fe 2 O 3 /BP2000 sample in the high frequency region shows the largest slop which indicates the Fe 2 O 3 /BP2000 separator has the fastest Li ion transportation speed which can accelerate the reaction speed [27][28].

Conclusion
In summary, the Fe 2 O 3 /BP2000 was successfully prepared by a simple wet-impregnation followed by a high temperature treatment in N 2 atmosphere.XRD, SEM and TEM analysis demonstrate that Fe 2 O 3 nanoparticles with a diameter of 5-30 nm are homogeneously embedded in the pores of BP2000.Because of the rich porous structure of BP2000 as well as the abundant metallic active sites, the BP2000/S cathode electrode, Li-S electrolyte (1 M LiTFSI with 0.1 M LiNO 3 dissolved in DOL/DME solvent), Fe 2 O 3 /BP2000 modi ed separator and Li anode electrode are assembled into Li-S coin cells in an Ar lled glove box.The cycle and rate performances of the Li-S cells were tested by a LAND battery testing equipment, The cyclic voltammetry (CV) pro les and electrochemical impedance spectroscopy (EIS) curves were conducted via a VMP 3 electrochemical workstation.

Figure 2 (
Figure 2 (a) and (b) show the SEM images of BP2000 and Fe 2 O 3 /BP2000 composite.The morphology of the two SEM images are approximately the same, demonstrating that Fe 2 O 3 are embedded in the pores of BP2000.In order to further investigate the micro structure of Fe 2 O 3 /BP2000 composite, TEM analysis are carried out and the TEM images are shown in Fig. 2 (c)-(f).Figure 2 (c) shows the TEM image of BP2000,

Figure 2 (
c) shows the TEM image of BP2000, the BP2000 nano particles are approximately 20-50 nm with abundant porous structure.Figure2(d) and(e) shows that Fe 2 O 3 nanoparticles are homogeneously dispersed in the pores of BP2000, furthermore, the HRTEM in Fig.2(f) clearly shows the lattice fringe of 0.25 nm which matches with the (311) face of Fe 2 O 3 nanoparticles, which demonstrates that Fe 2 O 3 nanoparticles are successfully embedded into the pores of BP2000.

Figure 6
Figure 6(b) shows the long term cycle performances of different batteries.When the sulfur area density in the cathode is 3 mg cm − 2 , the initial discharge capacities of PE, BP2000 and Fe 2 O 3 /BP2000 separator batteries at 0.1 C are 986, 1280 and 1350 mAh g − 1 respectively, as the current rate adds up to 0.5 C, the discharge capacities of the batteries are 650, 911 and 1140 mAh g − 1 respectively.At the 200 cycles, the discharge capacity of the PE separator battery suffers a sharp decline and the battery can hardly release capacity.By comparison, the BP2000 and Fe 2 O 3 /BP2000 separator batteries show a steady cycle