Enhanced Adsorptions to Polysulfides on Graphene-Supported BN

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Enhanced Adsorptions to Polysulfides on GrapheneSupported BN Nanosheets with Excellent Li-S Battery Performance in a Wide Temperature Range Ding Rong Deng, Fei Xue, Cheng-Dong Bai, Jie Lei, Ruming Yuan, Ming Sen Zheng, and Quan Feng Dong ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b05534 • Publication Date (Web): 25 Oct 2018 Downloaded from http://pubs.acs.org on October 27, 2018

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Enhanced Adsorptions to Polysulfides on GrapheneSupported BN Nanosheets with Excellent Li-S Battery Performance in a Wide Temperature Range Ding Rong Deng, Fei Xue, Cheng-Dong Bai, Jie Lei, Ruming Yuan, Ming Sen Zheng* and Quan Feng Dong*

State Key Laboratory for Physical Chemistry of Solid Surfaces, Department of Chemistry, College of Chemistry and Chemical Engineering, iChem (Collaborative Innovation Center of Chemistry for Energy Materials) Xiamen, Fujian, 361005, China

KEYWORDS : high temperatures, low temperatures, synergistic effect, Li-S battery, enhanced adsorptions

ABSTRACT

For Li-S batteries, the catalysis for S redox reaction is indispensable. A lot support material of sulfur electrode with multi-functional have been investigated widely. However, most of these

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studies were carried out at a room temperature, and the interaction between different component in matrix is not often paid enough attentions. Here, we report a graphene supported BN nanosheets composite in which the synergistic effect between BN and graphene greatly enhanced the adsorption for polysulfides, thus leading excellent performance in a wide temperature range. When used as a host materials of sulfur, it can make the Li-S bettery apply to a wide range of temperatures. From -40 ℃ to 70 ℃ , delivering a high utilization of sulfur, an excellent rate capability and outstanding cycling life. The capacity can stabilized at 888 mAh g-1 at 2 C after 300 cycles with a capacity attenuation less than 0.04% per cycle at 70℃. And the battery can deliver a capacity above 650 mAh g-1 at -40 ℃.

As the capacity of traditional lithium-ion batteries reaching a bottleneck, it is crucial to find new battery systems with higher theoretical capacity.1-3 Lithium-sulfur battery is known as the most promising lithium secondary battery, because of the highest theoretical capacity (1675 mAh g-1) of sulfur in solid materials.4-6 Li-S battery has aroused wide attention as advanced energy storage for portable electronics, electric vehicles (EV), hybrid electric vehicles (HEVs), and largescale stationary electric energy storage.7-17 As we all know, the catalysis for S redox reaction is necessary to Li-S batteries. Such a catalysis is usually achieved by adsorption of active species by the host materials.18-25 Therefore, many researches have been embarked on using so-called multi-functional matrix which can catalyze the reaction process of sulfur as the host materials in a Li-S battery. 26-34 However, these studies were mainly carried out at a room temperature. Actually, most of electronic equipments including EV or HEV need to be used in a wider range of temperature


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which both in high and low temperatures.35,36 For all the lithium batteries, it is a common phenomenon the capacity will be greatly attenuated under 0℃. Generally, conventional LIBs at 20 ℃ only retained approximately 20% of the room temperature capacity.37-39 The situation is more serious in lithium-sulfur batteries because of the complex reaction mechanism. For lithiumsulfur batteries, the discharge reaction can divide into two parts, one is correspond to the reduction of elemental sulfur to the long-chain LiPSs which is a liquid reaction process and the other one is the process of LiPSs formation to Li2S which is a solid reaction process. Compared to the intercalation mechanism of common lithium-ion batteries, the electro-chemistry of liquid reaction is relatively easy to react at low temperature. But the solid reaction process is more difficulty because of the slower kinetic velocity and lower reaction voltage. The low temperature will slows down the already unpleasant kinetic speed and makes the more severe polarization lead to more difficult solid reactions and even the solid phase reaction cannot be performed especially at a high current density. There are few researches were focused on the performance of Li-S batteries at low temperature. Cui et al. used a novel multifunctional graphene oxide-Zn(II)triazole to firmly entrap polysulfide by chemical interaction.40 The host material can improved Li-S performance at both room and low temperature. The liquid reaction process at low temperature has been exactly promoted, unfortunately, the solid phase reaction is still can not proceed. The discharge capacity in -20℃ was only 315 mAh g-1 at 0.5 C after 100 cycles. On the other hand, for lithium-sulfur batteries, high temperatures will promote lithium polysulfide (LiPS) dissolution into the electrolyte, resulting in worse cycle life.41-43 The possible approach has been developed to solve this issue by capturing sulfur within the host materials or interlayer. Sun et al. used a molecular layer deposited (MLD) alucone coating the carbon−sulfur electrodes.43 The cell showed a reversible capacity of 573 mAh g−1 at 0.1 C after 300 cycles in


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55 ℃. These problems cause a huge difficult which applying lithium-sulfur batteries to a wide range of temperatures. Herein, we found that there is a strong synergistic effect between graphene and BN nanosheets on the adsorption of polysulfides, especially under extreme temperatures. By implanting BN on graphene, a graphene-supported BN nanosheets (BN/graphene) composites was been synthesized. As isoelectronic species, BN and graphene have similar properties in configuration. The graphene-supported BN nanosheet composites thus possess an alternative surface electronic structure leading to a very different adsorptional characteristic. The synergistic effect between these two materials greatly enhanced the adsorption to polysulfides on a wide temperature range compared to pure graphene and BN nanosheets. Then the BN/graphene composites were used as the host materials for Li-S batteries, the reaction dynamics of sulfur redox could be accelerated, which will be the preconditions for obtaining a low temperature performance. At the room temperature, it can release a high capacity of 1553 mAh g-1 under 0.1 C, and remain a reversible capacity above 700 mAh g-1 at 5 C. When the temperature was -20 ℃ , the capacity was up to 1000 mAh g-1 at 0.1 C, and it could remain at about 500 mAh g-1 at 1 C after 300 cycles. Even the temperature was drop to -40 ℃ , the battery can still conduct normal charge/discharge and deliver a capacity above 650 mAh g-1. For the plagued high temperature problem in Li-S batteries, the shuttle effect was suppressed efficiently by the enhanced adsorption to LiPSs. The capacity can stabilized at 888 mAh g-1 at 2 C after 300 cycles with a capacity attenuation less than 14% (0.04% per cycle) at 70℃. RESULTS AND DISCUSSION


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X-Ray Diffraction (XRD) was used to analyze the composition of the as-prepared sample. As shown in Figure 1a, there is a obvious peak appears at 26.6° and three weaker peaks lie at 41.5°, 54.8° and 75.7°, which is correspondenced the (002), (100), (004) and (110) crystal planes of the standard BN crystal (JCPDS No. 85-1068). And the broad peak at around 26° is attributed to the graphene in the sample. The N2 adsorption–desorption isotherms was used to measure the pore size distribution and the specific surface area of the sample as shown in Figure 1b. The BN/graphene sample exerts a lagre specific surface area of 157.18 m2 g-1, and a high pore volume of 0.38 cm3 g-1. The average pore size of the sample is 4.5 nm, indicating that the asprepared sample is a mesoporous material with rich pore structure. The EDS in Figure S1 indicated that the graphene in the composite is about 36.8% wt, the BN is 54.8% and the O in the composite is about 7.7%. The morphology and structure characteristics were characterized by SEM and TEM images. Figure 1c and d are the SEM images of the overall view for the BN/graphene sample at different magnification. The BN nanosheets were evenly distributed on the surface of the graphene, and compared to the pure BN sample which was synthesised by the similar method (Figure S2), there is no obvious agglomeration. With the addition of graphene, BN preferentially adheres to the surface of graphene rather than to be aggregation. The two 2D materials, BN nanosheets and graphene, were integrated into 3D composites with a larger superficial area and more porous structure. (Figure S2 and S3). The TEM shows that the BN nanosheet with a thickness of 10 nm is uniformly arranged on the surface of graphene. The fingerprint of the BN crystal showed in Figure 2f is conformed to be the (100) plane. The element distribution mapping also manifested that BN nanosheets are evenly distributed on the graphene (Figure S4).


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The elemental sulfur and BN/graphene was combined at 155 °C by melt diffusion. TGA shows that the content of sulfur in the composites was 75.4% (Figure S5). Then the BN/graphene-S composites electrode was operated at a voltage range from 1.7 to 2.7 V to evaluate the electrochemical properties at room temperature. Figure 2a and b show the rate capability of the cell at different current densities and the discharge/charge curve corresponding to it. At the rate of 0.1C, the BN/graphene-S electrode can liberate a large specific capacity of 1553 mAh g−1, which is equivalent to 93% of the theoretical capacity of S. And the reversible charge capacity at this rate is 1541 mAh g−1, illustrating that almost all the discharge products are oxidized back, the coulombic efficiency is as high as 99.1%, showing an excellent utilization of sulfur at both discharge and charge process. At the current densities of 0.2 C, 0.5 C, 1 C and 2 C the discharge capacities for BN/graphene-S electrode are 1324 mAh g−1, 1178 mAh g−1, 1047 mAh g−1 and 914 mAh g−1, respectively, with high coulomb efficiency at the same time. A discharge capacity can be maintained at about 800 mAh g−1, even the rate increases to 5 C. The discharge capacity can recovered at 1400 mAh g−1 under 0.1 C after a high rate cycling, which is close to the capacity obtained at the beginning. All the charge/discharge curve at different current densities show two standard discharge platforms, which attributed to S8 to Li2S4 and further reduction of Li2S4 to Li2S2/Li2S, respectively. At the lower rate below 1 C, the first discharge plateaus almost release the total capacity what this stage should be, and even under the high rate, the capacity of the first discharge stage is also at a very high level. Besides the outstanding rate performance and high utilization of sulfur, the BN/graphene-S also demonstrated an excellent cycling performance. Figure 2c shows that the capacity and coulomb efficiency vs cycle number of the cell at 0.1 C. After 30 cycle, the discharge can still keep at above 1400 mAh g−1 with a coulomb efficiency close to 100%. Under the current density of 0.5 C and 1 C, BN/graphene-S delivered a discharge


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capacity about 970 mAh g−1 and 900 mAh g−1 after 100 cycles. When the current density rise to a high rate at 2 C, the reversible capacity finally stabilized at 730 mAh g−1 after 500 cycles with a low capacity decay rates of 0.02% per cycle, which indicates the extremely high cycling stability. The BN/graphene-S cell also shows decent electrochemical performance at high areal sulfur loading. When the areal sulfur loading upgrade to 4.1 mg cm-2, the cell was able to deliver high capacity of 1231 mAh g−1, 1124 mAh g−1, 988 mAh g−1, and 877 mAh g−1 at a current density of 0.05C, 0.1 C, 0.2 C and 0.5 C, which corresponded to an areal capacity of 5.0 mAh cm−2, 4.9 mAh cm−2, 4.0 mAh cm−2, and 3.6 mAh cm−2 (Figure S6). And after 100 cycles, a capacity of 730 mAh g−1 and 670 mAh g−1 can still be maintained stably at 0.5 C and 1 C, respectively, indicating outstanding cycling stability even with such a high sulfur loading. The high cycling stability of the cell indicates BN/graphene can reduce the shuttle effect of LiPSs effectively which may attribute to the stronge adsorption between the BN/graphene and LiPSs. Figure 3a is a visual discrimination to manifest the adsorbability of BN/graphene, the pure BN nanosheets and graphene were used as contrast. At first, 30 mg graphene, BN and BN/graphene were added into the 10 mM Li2S6 solution, respectively. After 3h, the color of solution with BN became light and there was alomost no change of the solution with graphene. This indicates that BN has a stronger adsorption effect on polysulfides compared to graphene. What`s interesting is that the solution with BN/graphene became colourless after 3h, which implys there are obvious synergies between BN nanosheets and graphene resulting to an enhanced adsorption to LiPSs. When the standing time has been extended to 12 hours, the solution with BN nanosheets becomes gradually almost colourless, while there was no further lighter of the solution with pure graphene. The phenomenon indicates that the adsorption mainly comes from boron nitride in the composite, and this adsorption can be enhanced significantly by


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the synergistic effect between BN and graphene. To understand the mechanism of the adsorption of BN/graphen, different ion adsorption was examined (Figure 3b and c). Co3+ and MnO4− were chosen as the typical examples of positive and negative ions. The UV spectra was used test the liquid supernatant after adsorb for 12 h. Compare with the blank solution of Co3+, both the BN nanosheets and BN/graphene can effective adsorption the positive, and the pure graphene has nearly no adsorption effect for Co3+; a similar situation occurs in the adsorption experiment of negative ions. The UV spectra indicated that the BN has an effective adsorption for both positive and negative ions. To understand why the addition of BN could enhance the adsorption of polysulfides, we perform the electrochemical deposition of Li2S with current density of 2.5 μA/cm2 (Specific steps were shown in experimental section). Surprisingly, it is found that most of discharged products deposit at the edges of the BN nanosheets, as shown in Figure S7. This indicates that the active sites of the electrocatalyis would be at the edges of the BN nanosheets. X-ray photoelectron spectroscopy (XPS) analysis shows that there were two types of boron species (Figure S8); one was coordinated with nitrogen on the flat surface whereas the other was bonded to oxygen at the edge. Recently, Hermans et al. suggested that the armchair edges of BN nanosheets might constitute the >B-O-O-N< groups.44 Similarly, we build a symmetric periodic BN belt by exposing the armchair edges. We assume that on each edge, there existed a >B-O-O-N< moiety, and the other B and N atoms at the edge are saturated by H atoms, see model A in Figure S9. However, our density functional theory (DFT) calculations show that such a model might be unstable under the charge/discharge condition because the O-O bond would readily react with the Li+ + e to form O-Li bonds. Computationally, the reaction of >B-O-O-N< + 2Li+ + 2e  >BO-Li + >N-O-Li was predicted to be strongly exothermic by 11.2 eV. Thus，we propose that the


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more reasonable model would be model B which consists of O-Li bonds, see Figure S10. It should be noted that Shi et al. also proposed that >B-O-H groups would be generated in situ during the oxidative dehydrogenation of light alkane,45 lending some support to our model. Table 1 shows the adsorption energies of polysulfide species on the model B. And the adsorption energies on the terrace of BN and graphene sheets are also listed for comparison. The optimized adsorption structures are depicted in Figure S10. For different adsorption sites, the general trends can be summarized as follows: (1) for all the adsorption structures we considered, Li2Sn would interact with the surfaces through their Li atoms; (2) for given site, the adsorption energies of Li2Sn generally increased with the increase of n; (3) for given n, the adsorption at the BN edge site is about 1.0 eV more stable than those on the terrace of BN and graphene, due to electrostatic attraction between O-Li and the S atoms directly coordinated to Li. All these finding come to the conclusion that edge sites favor the adsorption of polysulfides than the terrace sites do, nicely explaining why the addition of BN nanosheets would enhance the performance of electrocatalysis. Unfortunately, the agglomeration of the BN nanosheets is serious, leading to the very less amounts of adsorption site. For the BN/graphene composite, the BN nanosheets were unfolded on the graphene surface. Therefore, a lot of edge sites of the BN/graphene composite can be obtained and the enhanced adsorption achieved. Figure 3d is the charge/discharge curve of these three host materials at 0.1 C. It is obvious that the BN/graphene cell deliver the highest discharge capacity among them. The increased capacity was mainly from the second discharge platform, which suggests that BN/graphene can effectively catalyze the solid-phase reaction process of reduction of Li2S4 to Li2S. During the charge, the capacity and coulombic efficiency of BN/graphene was also the best one, which can be attributed to the enhanced adsorption of BN/graphene to LiPSs, thus catalyzing the reaction


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from Li2S to long-chain lithium polysulfides and then to S8 effectively. The electrical resistivity of BN/graphene is 7.7 x 10-3 Ω ·m measured by Powder resistivity tester (FT-02A), which is higher than graphene (6 x 10-4 Ω · m) but much lower than the BN nanosheets (6.25 x 105 Ω · m). Figure 3e shows the EIS of the different cathodes. Due to the relative poor electrical conductivity, the BN/graphene shows a little lager electrode resistance (Rs, the point at which the curve touches abscissa in the high frequency range) than the graphene cell. But the BN/graphene shows the lowest charge transfer resistance (Rct, the semicircle in the high to medium frequency range) among the three samples, indicating that the BN/graphene electrode possesses rapid electrochemical reaction kinetics, which is consistent with previous electrochemical data. Figure 3e is the discharge capacity of these three host materials under the current density of 1 C. The BN/graphene electrode shows the largest discharge capacity and highest cycling stability among them. Benefitting the 3D conducting network of graphene, the pure graphene cell also shows a high initial capacity but bad cycling stability. On account of adsorption of BN for LiPSs, the BN cell shows a better cycling stability than graphene yet, but the capacity is far less than the BN/graphene one. The N,B-doped graphene alos was synthesized and used as a control sample. The the content of N in the N,B-doped graphene was 15.05% wt and the content of B was 6.9% wt (Figure S11). The electrochemical performance of the N,B-doped graphene cell was given in the Figure S12. It showed that the N,B-doped graphene presented a better cycling stability than ordinary graphene, but It's still inferior to the BN/graphene one. Whether a battery can work in a wider range of temperature is one of the key factors in its application. Figure 4 is a visual discrimination to manifest the adsorbability of BN/graphene at a low-temperature (-40 ℃ ) and a high-temperature (70 ℃ ). The same as the test at room temperature, 10 mM Li2S6 solution was used as the representative of LiPSs. Due to the different


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solubilities of Li2S6 at different temperatures, the color of 10 mM Li2S6 solution was not same, the lower the temperature of the deeper the color. It is obvious that graphene has no adsorption for polysulfide at both high and low temperature, and the adsorption capacity of BN nanosheets is weaker than that of room temperature. Even after 12h, BN showed no obvious adsorption effects. On the contrary, due to the synergistic effect between BN and graphene, the BN/graphene shows excellent adsorption properties at both high and low temperature (Figure 4). It illustrates the the BN/graphene may greatly reduced the shuttle effects and accelerated the sulfur redox reaction rate at a wide range of temperatures. For the lithium-sulfur battery, the high-temperature will lead to more serious shuttle effects, resulting in worse cycle stability. Since BN/graphene had a strong and fast adsorption effect for LiPSs at high-temperature, it may enhance the stability of the batteries. Figure 5 shows the performance of BN/graphene for Li-S battery at 55℃, the ordinary carbon host material Sup P was used as the contrast. The charge/discharge curve is shown in Figure 4a and Figure S13. Generally, at a high temperature, the reaction rate of the battery is expedited. There is no obvious improvement of the capacity at 0.1 C, because of under the slowish rate, the battery reaction can completely response even at room temperature. At a high rate of 1 C, the capacity at 55℃ is higher than that at room temperature, and the increment of Sup P-S composite is larger than the BN/graphene-S one. This will be attributed to that the BN/graphene can catalyze the reaction process of the battery, the reaction is more complete at the room temperature than the Sup P-S. Although the capacity increment of BN/graphene-S is lower, the final charge and discharge capacity is higher than the Sup P-S electrode. A strict requirement for a hightemperature lithium-sulphur battery is the cyclic stability. The capacity vs cycle number at 1 C by the two host materials were shown in Figure 4b. For a Sup P cell , the initial capacity at 1 C is


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about 1000 mAh g−1, and the capacity fell to below 400 mAh g−1 after only 100 cycles with a high capacity fading rate more than 60%. To the contrary, the BN/graphene cell first deliver a discharge capacity at 1150 mAh g−1, and after 100 cycles the reversible capacity can still keep at about 1050 mAh g−1, the capacity fading rate was only less than 10% which displayed a outstanding cyclic stability at 55℃. Even the temperature rise to 70℃, the discharge capacity at 2 C still keep at about 888 mAh g−1 after 300 cycles with only a 14% capacity fading rate . These results illustrate that the BN/graphene host materials would lead an excellent cycling stability for Li-S batties at high temperature. Most lithium batteries do not work properly at low temperatures because of the slow reaction kinetics of the batteries. This situation becomes even more serious due to that the reaction process of Li-S battery is more complex with undergoing various phase changs. The BN/graphene showed a strong catalytic effect on the conversion of S at low temperature, thus improving the performance of the lithium-sulfur battery. The lithium-sulfur battery which use BN/graphene as host materials was tested under low temperatures as shown in Figure 6. Figure 6a is the charge and discharge voltage profiles at 0.1 C by the BN/graphene cell and Sup P cell at 0℃. Both the two cell showed a standard curve of the Li-S battery under the temperature. Due to the first discharge platform of Li-S battery is a liquid-phase process, the reaction is relatively fast at room temperature, even the Sup P without catalysis can release high capacity. But when the temperature drop to 0℃, the rate of liquid-phase reaction is also greatly reduced, Sup P can not speed up the reaction at this time, and the BN/graphene can accelerate the process which lead the obvious capacity improvement at both the first and second discharge plateau. The BN/graphene showed a much higher capacity and smaller polarization at 0℃ than the Sup P cell. When the current density is 0.1 C and 0.5 C, the discharge capacity of BN/graphene cell can attain at 1410


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mAh g−1 and 1180 mAh g−1, with a little less comparing with the capacity at room temperature. And BN/graphene cell delivered a high reversible capacity above 800 mAh g−1 after 100 cycles at 1 C under 0 ℃ which was amount to 90% of the cell delivered at room temperature. In comparison, the Sup P cell can only remained about 400 mAh g−1 after 200 cycles which was only a half of the BN/graphene one. When the temperature goes down to -20 ℃, the “death line” for ordinary lithium batteries, the charge/discharge curves had changed at this time (the cells were activated at 0.05 C for a cycle under -20℃ and -40℃). Here, the TiN tube host material, which shows an excellent performance for Li-S battery at room temperature22 and the pure S active material without host were used to be the contrast. At such a low temperature, the reaction rate of the battery becomes very slow, and the original liquid-phase reaction platform became into two parts, corresponding to S8 reduced to Li2S8 and further reduction to Li2S4, respectively.46 The polarization of battery was very large at -20 ℃, and the liquid-phase reaction of Li-S battery becomes very slow and the reaction of solid-phase is more difficult. At a small rate of 0.1 C, the pure S cell has been unable to carry out the solid-phase reaction, and the voltage platform of TiN and Sup P cell was got extremely low. The TiN tube had a certain catalysis for sulfur reaction at room temperature, but at -20 ℃ the catalysis was almost disappearing, the voltage platform of solid-phase reaction is only a little higher than the Sup P cell. It illustrates that the common host material which can get a good performance at room temperature does not work at low temperature. In contrast, the cell used BN/graphene as the host material showed the much smaller polarization. The voltage plateau of solid-phase reaction was at approximately 2.0 V, and liberated above 1000 mAh g−1 capacity at 0.1 C under -20 ℃. And under the larger rate of 0.5 C, most cell can’t proceed the solid-phase reaction. The BN/graphene cell still could occur a relative complete reaction of the sulfur reduction with a high


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discharge capacity of about 800 mAh g−1, which was amount to 75% of the cell delivered at room temperature. After 100 cycles, the reversible capacity was maintained at more than 700 mAh g−1, under the same condition, the discharge capacity of the Sup P, TiN and pure S cell can only deliver a value of approximately 200 mAh g−1, 250 mAh g−1, and 150 mAh g−1, respectively. Figrue S14 was the electrochemical impedance spectroscopy (EIS) of these four electrodes after cycles at -20℃. In the proposed circuit model, Re is the resistance of electrolyte, Rint//CPEint is the interphase resistance and its related capacitance, Rct//CPEct is the charge-transfer resistance and its related capacitance, and CPEdif is diffusion impedance that probably represents Li ions diffusion process. The first semicircle in the HF region is related to the interphase resistance of the SEI, the Rint became obvious at low temperature and there is no decisive difference among them at this section. But there is very different at the second semicircle in the MF region which is related to the charge-transfer resistance. The BN/graphene shows a much lower Rct than the other electrodes at -20 ℃ , indicating that the BN/graphene electrode possesses a much rapider electrochemical reaction kinetics. The BN/graphene cell still can deliver a capacity at 500 mAh g−1 after 300 cycles when the rate rise to 1 C (Figrue S15 shows the discharge/charge curve at 1 C). And even the temperature dropped to -40 ℃ , the BN/graphene still exhibited pretty electrochemical properties, the electrode can liberate the high capacityof 667 mAh g−1 and 487 mAh g−1 at 0.1C and 0.2 C. The morphological images of both electrodes after 100 cycles at 0.5 C under different temperatures were shown in Figure S16, S17 and S18. After long cycles, the lithium anodes from the BN/graphene cell at different temperatures are still bright, there was no macroscopic dissolved LiPSs on the surface. In contrast, the orange areas on the anode with Sup P as the host material is conspicuous even at room temperature, confirming that a mass of dissolved LiPSs is


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across to the lithium metal from the separator. And there were no obvious crack on the cathode electrodes from the BN/graphene cell at different temperatures which illustrate that the BN/graphene can inhibite the volume expansion of the electrode. It can be seen the graphene structure of the cathode electrodes has been still kept after 100 cycles, and the sulphur or LiPSs were cover on the host material. The SEM image of the lithium anodes show that the dendritic crystals are not particularly serious after 100 cycles at different temperatures. The capacity of the BN/graphene cell, with changing the temperature simultaneously during the cycling test at a rate of 0.5 C was shown in Figure 7. The cell shows a discharge capacity about 1285 mAh g−1 and 1148 mAh g−1 under 70℃ and 55℃, when the temperature change to room temperature, the cell liberate a specific capacity of 968 mAh g−1 which is similar to the performance tested directly at room temperature. As the temperature continues to drop, at the 0℃ , -20℃, and -40℃ the discharge capacities for BN/graphene-S electrode are 738 mAh g−1, 669 mAh g−1 and 401 mAh g−1. The discharge capacity can recovered at 912 mAh g−1 under 25 ℃ after a low temperature cycling, which is illustrate that the capacity can have an outstanding maintain after a period of time in a higher or lower temperature.

CONCLUSIONS In summary, a graphene-supported BN nanosheet composite was successful synthesised. Due to the synergistic effect between BN and graphene, a significant enhanced adsorption has been achieved with the BN/graphene composite which demonstrates high catalysis activity for both the discharge and charge process of Li-S battery in a wide temperature range. When the


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BN/grphene composites were used as the host material in lithium-sulfur batteries, it showed excellent electrochemical performance. More importantly, based on the enhanced adsorption, this host material can make the Li-S bettery apply to a low and high temperature. From -40 ℃ to 70 ℃ , the cell with BN/grphene material can deliver an excellent rate capability, outstanding cycling life and high utilization of sulfur. EXPERIMENTAL SECTION Synthesis of BN@graphene: All chemicals were analytical grade and used without further purification. 0.3 g boric acid was dissolved into 50 mL of deionized water. Then 6.9 g carbamide was added into the solution under stirring. After stirring for 12 h, 60 mg graphene was added into the solution, keep stirring for a few hours, the mixture was then transferred to dried at 60 °C in vacuum. Then the sample was calcined at 900 °C in an nitrogen atmosphere for 5 h, after being cooled to room temperature, BN@graphene was thus obtained. Bare BN nanosheets ware prepared at the same condition without adding graphene. The N,B-doped graphene was synthesized following a method reported in the literature.47 Preparation of the Sulphur composites: The BN/graphene-S composite was prepared via the classical melt-diffusion method. First, the required amount of elemental sulfur and BN@graphene (3:1 by mass) were mixed thoroughly by grinding. Then the mixture was heated at 155 °C for 6 h and cooled to room temperature. The other S composite was prepared by the same method. Adsorption Sample Preparation: The 10 mM Li2S6 solutions were prepared by dissolving appropriate amounts of Li2S6 into 5 mL of DOL/DME solvent in five vials with the ame volume.


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5 mg/mL Sup P, BN nanosheets, graphene and BN@graphene were then added to there different vials, respectively. All procedures were completed in an Ar-filled glovebox. The other adsorption sample preparation was prepared by the same method in addition to replacing the Li2S6 with CoCl3 or KMnO4. Characterization of the materials: The crystal structure of the samples was using X-ray diffraction (XRD, Rigaku Ultima IV) with Cu Ka radiation (λ = 0.1548 nm) to measurement. The morphology of samples was gained from Field-emission scanning electron microscopy (SEM Hitachi S-4800, 15 KV) and transmission electron microscopy (FEI Tecnai F30, 300 kV). Thermogravimetric (TG) analysis was carried out on a Perkin Elmer instrument. And the specific surface

areas

and

adsorption/desorption

pore using

size

distribution

with

a

plot

were

Micromeritics

measured

Tristar

II

by

3020

N2

isothermal

analyzer.

The

electrical resistivity of the samples was using Powder resistivity tester (FT-02A). Electrochemical measurements: To evaluate the electrochemical properties of the BN/graphene-S composite as a anode material, its electrochemical testing was conducted. The anode electrode consists of 70 wt % active material (sulfur composites such as BN/graphene-S, BN-S, graphene-S, Sup P-S, TiN-S and pure S), 20 wt % Super P, and 10 wt % binder (watersoluble polymer n-lauryl acrylate). All test cells were assembled in an Ar-filled glovebox. Celgard 2400 was used as the separator, Li foils were used as the counter electrode, and the electrolyte was 0.5 M LiCF3SO3 and 0.5 M LiNO3 (dissolved in DME and DOL in a 1:1 volume ratio). The areal mass loading of sulfur in the electrode was about 1.5 mg cm−2, and 30 μL of electrolyte was added into the CR2016-type coin cells. The galvanostatic charge-discharge cycling was performed with a voltage window of 1.7-2.7 V. The cell showed no activation when current densities under 1 C. The cell was activated at a 0.1 C rate for 3 cycles when the current


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densities were 1 C and 2 C. And the cell was activated at a 0.05 C rate for a cycle when the temperature was -20℃ and -40℃. Electrochemical deposition of Li2S: A electrolytic tank, with the BN nanosheets on graphite flake as cathode, Li metal as anode, and 0.5 M Li2S8 polysulfide solution (based on S) as catholyte, was used to perform the electrochemical deposition with current density of 2.5 μA/cm2 at 1.7 V for 1h. COMPUTATIONAL DETAILS Periodic DFT calculations are performed with the Vienna ab initio simulation package (VASP) using projector augmented waves (PAW) pseudopotentials and the exchange-correlation functionals parametrized by Perdew, Burke, and Ernzerhof (PBE) with a cutoff energy of 400 eV. To model the edge sites of BN, we firstly construct a symmetric belt containing 7 × 5 BN hexagons which exposes armchair edges on both sides. We assume that on each side, one O-O is added across one of >BN< groups to form the >B-O-O-NB-O-O-N< would be unstable upon charging/discharging as the O-O bond would be broken and the O-Li bonds would be formed, resulting in >B-O-Li and >N-O-Li, c.f. model B in Figure S9. To simulate the terrace sites, 6 × 6 BN and 6 × 6 graphene sheets are also constructed. All the layers and the adsorbates were fully relaxed. For all of the calculations, the vacuum regions between the layers were more than 15 Å, and Monkhurst-Pack k-point sampling with approximately 0.05 × 2π Å-1 spacing in a reciprocal lattice is utilized.


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Here we consider the solvent effects for polysulfide species because they are solvated before deposition. The solvation free energy ΔGsolv was obtained with the level of M06-2X/6-31+G(d,p) by using the SMD solvation model in 1,4-dioxane solvent. The calculated formula is listed in following[ref J. Phys. Chem. B 2011, 115, 14556–14562; J. Phys. Chem. B 2009, 113, 6378– 6396]: ΔGsolv =ΔGENP + GCDS + ΔGconc

(1)

Here, ΔGENP denotes the difference of electronic (E), nuclear(N), and polarization (P) components of the free energy from gas-phase to liquid-phase. GCDS is the contribution from cavitation, dispersion, and solvent structural effects. ΔGconc denotes the free energy correction from the gas-phase standard state (1 atm) to the solution-phase standard state of 1 M, which is 1.89 kcal/mol at 298.15 K. The solvation corrected energies (Ead@liquid) for the Li2Sn can be approximately written as: Ead@liquid = Ead -ΔGsolv

(2)

In which Ead@liquid, Ead and ΔGsolv represent as the solvation corrected energy of adsorbates, the energy of adsorbate from VASP calculation, and the solvation free energies from Gaussian09. Thus, the adsorption energies with solvation correction (ΔEads) can be defined as: ΔEads = Ead/surf - Ead@liquid - Esurf

(3)

Here, Ead/surf, Ead@liquid and Esurf were the total energies of the optimized adsorbate/surface system, the adsorbates in the solution, and the surface, respectively.


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FIGURES

Figure 1. (a) XRD patterns of BN/graphene. (b) N2 adsorption-desorption isotherm loop and pore-size distribution plots of BN/graphene. (c, d) SEM images of BN/graphene. (e)TEM images of BN/graphene. (f) HRTEM image of the BN/graphene.


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Figure 2. (a) (b) Rate capability and corresponding Charge and discharge voltage profiles of the BN/graphene at different current densities. (c, d, e) Charge and discharge capacity and Coulombic efficiency versus cycle number at current densities of 0.1 C, 0.5 C, 1 C and 2 C.


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Figure 3. (a) Sealed vials of a lithium polysulfide solution (Li2S6 dissolved in DOL/DME solvents) containing graphene, BN, BN/graphene and blank electrolyte quiescence after 3 h and 12 h. (b)(c) UV spectra of Co3+ solution and MnO4- solution after adsorption. (d) First charge and discharge voltage profiles of BN/graphene, graphene and BN electrodes at 0.1 C. (e) Electrochemical impedance spectra of the three electrodes. (f) Discharge capacity of BN/graphene, graphene and BN versus cycle number at a current density of 1 C.


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Figure 4. Sealed vials of a lithium polysulfide solution (Li2S6 dissolved in DOL/DME solvents) containing graphene, BN, BN/graphene and blank electrolyte quiescence after 3 h and 12 h at 40℃ (a) and 70℃ (b).


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Figure 5. (a) First charge and discharge voltage profiles of BN/graphene and Sup P at 1 C under room temperature and 55℃, respectively. (b) Discharge and charge capacity of BN/graphene and Sup P versus cycle number at a current density of 1 C under 55 ℃ . (c) Charge and discharge capacity versus cycle number at current densities of 2 C under 70℃.


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Figure 6. First charge and discharge voltage profiles of BN/graphene and Sup P under 0.1 C (a) and 0.5 C (b) at 0℃, respectively. (c) Discharge and charge capacity of BN/graphene and Sup P versus cycle number at a current density of 1 C under 0℃. First charge and discharge voltage profiles of BN/graphene, Sup P, TiN and withou host at 0.1 C (d) and 0.5 C (e) under -20 ℃ , respectively. (f) Discharge capacity of BN/graphene, Sup P, TiN and withou host versus cycle number at a current density of 0.5 C under -20℃. (g) Charge and discharge capacity versus cycle number at current densities of 1 C under -20℃. (h) First charge and discharge voltage profiles of BN/graphene at 0.1C, 0.2C and Sup P under 0.1 C at -40℃, respectively.


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Figure 7. The charge and discharge capacity of the cell with BN/graphene host materials changing the temperature simultaneously during the cycling test at a rate of 0.5 C.

Table 1. The adsorption energies of Li2Sn at the edge and on the terraces of BN nanosheet and graphene (Unit: eV). Adsorption site

Li2S8

Li2S6

Li2S4

the edge of BN

-1.80

-1.66

-1.72

the terrace of BN

-0.73

-0.69

-0.57

the terrace of graphene

-0.62

-0.56

-0.47


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publication website. Additional XRD,SEM, EDX, BET, TG and electrochemical performance (PDF) The authors declare no competing financial interest.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected] ACKNOWLEDGMENT We gratefully acknowledge the financial support from the National 973 Program (2015CB251102), the Key Project of NSFC (21673196, 21621091, 21703186, 21773192), the Fundamental Research Funds for the Central Universities (20720150042, 20720170101).

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Chem. Phys. 2015, 17, 25440-25448.


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For Table of Contents Only


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Enhanced Adsorptions to Polysulfides on Graphene-Supported BN

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