Co4N Nanosheet Assembled Mesoporous Sphere as a

Jun 1, 2017 - N Nanosheet Assembled Mesoporous. Sphere as a Matrix for Ultrahigh Sulfur Content. Lithium−Sulfur Batteries. Ding-Rong Deng, Fei Xue, ...
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Co4N Nanosheet Assembled Mesoporous Sphere as a Matrix for Ultrahigh Sulfur Content Lithium−Sulfur Batteries Ding-Rong Deng, Fei Xue, Yue-Ju Jia, Jian-Chuan Ye, Cheng-Dong Bai, 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 S Supporting Information *

ABSTRACT: High utilization and loading of sulfur in cathodes holds the key in the realization of Li−S batteries. We here synthesized a Co4N mesoporous sphere, which was made up of nanosheets, via an easy and convenient method. This material presents high affinity, speedy trapping, and absorbing capacity for polysulfides and acts as a bifunctional catalysis for sulfur redox processes; therefore it is an ideal matrix for S active material. With such a mesoporous sphere used as a sulfur host in Li−S batteries, extraordinary electrochemistry performance has been achieved. With a sulfur content of 72.3 wt % in the composite, the Co4N@S delivered a high specific discharge capacity of 1659 mAh g−1 at 0.1 C, almost reaching its theoretic capacity. Also, the battery exhibited a large reversible capacity of about 1100 mAh g−1 at 0.5 C and 1000 mAh g−1 at 1 C after 100 cycles. At a high rate of 2 C and 5 C, after 300 cycles, the discharge capacity finally stabilized at 805 and 585 mAh g−1. Even at a 94.88% sulfur content, the cathode can still deliver an extremely high specific discharge capacity of 1259 mAh g−1 with good cycle performance. KEYWORDS: Li−S battery, mesoporous sphere, cobalt nitride, adsorb, catalyze, high sulfur loading

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To deal with these challenges, great efforts have been made to optimize the composition and the structure of the sulfur cathode; most of these combined sulfur with different kinds of host materials.17−29 However, with the accretion of the host materials, the content of S in the active material of the cathode decreases accordingly. Therefore, finding a balance between the content of S and good electrochemical performance plays a significant role in the study of the Li−S battery. Most of the host materials attach S with different kinds of conductive additives, such as various types of carbonous materials. Nazar et al. used porous carbon materials (CMK3) as sulfur host materials, which improved the cycling life and specific capacity for their lithium−sulfur battery.17 When the content of S at the active material of a cathode was about 70%, the discharge capacity of the first cycle was 1380 mAh g−1 at a current density of 0.1 C (1 C = 1675 mA g−1). Carbon materials can promote the electrical conductivity of the cathode and act as good buffers for the large volume expansion of sulfur during the

here is no doublt that energy demand increases steadily with time because of the global population increase and economic growth. The search for advanced energy storage systems with higher energy densities is very important for powering our future communities.1−5 The lithium−sulfur (Li−S) battery has been one of the best devices for nextgeneration high-energy storage systems as a result of its high theoretical capacity. The lithium−sulfur battery has a high energy density and theoretical capacity of 2500 kW kg−1 and 1675 mAh g−1, respectively, far superior to the current lithiumion batteries.6−11 However, with years of development, the Li− S battery is still not primed for commercialization due to its incomplete sulfur utilization, capacity attenuation, poor cycling life, and low Coulombic efficiency. These drawbacks are attributed to the poor conductivity of sulfur and the emergence of the discharging products (Li2S and Li2S2) and the dissoluble lithium polysulfide (Li2Sn) in the charging and discharging processes.8,12,13 During back and forth shuttling in the electrolyte between the cathode and anode, polysulfide lithium triggers a polysulfide-shuttle process, which leads to battery capacity attenuation, poor cycling life, and low Coulombic efficiency.14−16 © 2017 American Chemical Society

Received: March 21, 2017 Accepted: June 1, 2017 Published: June 1, 2017 6031

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ACS Nano charge−discharge process.17−23 On the other hand, host materials that bind polysulfides through chemical interactions have been discovered and applied as cathode electrodes for Li− S batteries.24−31 Nazar et al. reported that Ti4O7 can be used as an effective LiPS absorbent.24 When the content of S in the Ti4O7/S composite was about 60%, the discharge capacity of the first cycle was 1200 mAh g−1 at a rate of 0.05 C, and it exhibited a reversible capacity of about 850 mAh g−1 at 0.5 C after 100 cycles. After that, polar materials such as metal oxides, metal nitrides including Ti4O7,24,25 TiO2,26 MnO2,27,28 MnO,30 and TiN,31,32 and functionalized carbon materials such as nitrogen-doped carbon/graphene15,29 have been reported to be used as effective lithium polysulfide (LiPS) absorbents. These host materials in cathodes will reduce the “shuttle effect” and improve the cycle stability of the lithium−sulfur battery. Unfortunately, according to the results of existing reports, the sulfur content of the active material in Li−S battery cathodes is below 70 wt %, which limits commercialization. In order to achieve higher sulfur content, some host materials have been employed in a recent study.33−36 However, the high loading of elemental S will lead to a more serious shuttle effect, rapid capacity loss, lower specific capacity, and poor rate properties. Zheng et al. used the hierarchical porous carbon rods as sulfur host materials. When the sulfur content was increased to 80%, it showed a first-cycle discharge capacity of 972 mAh g−1 at 0.5 C and 800 mAh g−1 after 30 cycles at 1 C.36 In recent papers, high area loadings of S of about 10 mg cm−2 have been reported.37−41 Therefore, increasing the utilization of sulfur in cathodes at high S content continues to be a great challenge for the Li−S battery. Here, a simple and efficient method for synthesizing a Co4N mesoporous sphere composed of nanosheets was reported. The Co4N mesoporous spheres were uniformly distributed and displayed a porous structure. In comparison with the Co3O4 mesoporous sphere, which has a similar pore size distribution and specific surface area, the Co4N mesoporous sphere exhibited not only higher affinity for polysulfide but also stronger capability for polysulfide adsorption than the Co3O4 sphere. Furthermore, the Co4N mesoporous sphere demonstrated bifunctional catalytic activities for the Li−S battery. The Co atoms in Co4N can promote the discharge process and N atoms can improve the charge process of the lithium−sulfur battery (Scheme 1). As the Co4N mesoporous sphere was used

as sulfur host materials for the Li−S battery, it displayed remarkable cycling performance, especially in the very high sulfur content regime. When the weight ratio of elemental S and Co4N spheres was 3:1 (S content as high as 72.3 wt %), the battery exhibited a large reversible capacity above 1100 mAh g−1 at 0.5 C and 1000 mAh g−1 at 1 C after 100 cycles. At a high rate of 2 C, it showed a high capacity retention of above 94% with a capacity decay of 0.01% per cycle. After 300 cycles, the discharge capacity finally stabilized at 805 mAh g−1. Even with a weight ratio increased to 9:1 (S content of 89.64 wt %), the Co4N−S composite still delivered an outstanding rate performance and cycling stability (801 mAh g−1 after 100 cycles at 1 C and 670 mAh g−1 after 300 cycles at 2 C). More importantly, even after the weight ratio of S and Co4N spheres was adjusted to 19:1, the Co4N/S composite maintained a high cycling stability (640 mAh g−1 after 100 cycles at 1 C).

RESULTS AND DISCUSSION XRD analysis was used to characterize the as-prepared samples, and the results are shown in Figure S1. Figure S1a shows that the sample is pure Co3O4, all the peaks can be assigned to the standard Co3O4, and no other contaminations were observed, indicating a high purity of the Co3O4 crystalline phase. The XRD patterns achieved after a simple nitridation reaction in Figure S1b match well with those of the pure Co4N phase with a face centered cubic (fcc) structure, indicating that the Co3O4 can be successfully converted into a Co4N product.42 The energy dispersive spectrometer in Figure S2 shows that the atomic ratio of Co and N is about 4:1. Field-emission scanning electron microscopy images provide insight into the structure and morphology of the as-prepared Co3O4 and Co4N. Figure 1a and b are the scanning electron

Scheme 1. Schematic illustration of a Co4N sphere and its interaction with LiPSs during the discharge/charge process of the lithium−sulfur battery.

Figure 1. SEM images of the Co3O4 phase (a, b) and Co4N phase (c, d). HRTEM image of the Co4N is presented in the inset of (d).

microscopy (SEM) images of the overall view of the Co3O4 power; it shows a uniform distribution of a spherical morphology with a diameter of 2−3 μm. Figure 1b shows that the Co3O4 spheres display a porous structure consisting of Co3O4 nanosheets with a 10−20 nm thickness. Figure 1c and d illustrate the SEM images of the products obtained after thermal treatment of Co3O4 at 400 °C for 4 h in an ammonia atmosphere. The images reveal that most of the spheres are retained, and the surface morphology of the porous structure is mostly maintained after calcination, except for some of the 6032

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Figure 2. N2 adsorption−desorption isotherm loop and pore size distribution plot of the Co3O4 and Co4N phases.

Figure 3. Sealed vials of a lithium polysulfide solution (Li2S6 dissolved in DOL/DME solvents) containing Sup P, Co3O4 phase, Co4N phase, and blank electrolyte quiescence after 3 h (a) and 12 h (c). Raman spectra of a blank solution without Li2S6, Li2S6 solution, Li2S6 solution with Sup P, Co3O4 phase, and Co4N phase after 3 h (b) and 12 h (d), respectively.

stability of the lithium−sulfur battery. To further illustrate the affinity for polysulfide, a visual discrimination has been used and the Li2S6 was utilized as the representative polysulfide. Co3O4 spheres, Co4N spheres, and Super P with the same quality were first added into a solution of Li2S6 in dioxolane (DOL)/dimethoxyethane (DME) solvent. As shown in Figure 3a, the excellent intrinsic capability of Co4N spheres to adsorb Li2S6 was obvious. After adding Co4N spheres, the yellow Li2S6 solution completely changed to colorless after 1 h. After 3 h, the color of the Li2S6 solution with Co3O4 spheres became lighter but was not completely obliterated, whereas that after adding Sup P remained yellow, indicating that there is still a large amount of Li2S6 in the solution. This showed that although the Co3O4 spheres have a higher specific surface area than Co4N spheres, Co4N spheres exert a stronger and faster adsorption for polysulfide. Raman spectroscopy was used to measure the blank solution (DOL/DME), Li2S6 solution, Li2S6 solution with added Sup P, Li2S6 solution with added Co3O4 spheres, and Li2S6 solution with added Co4N spheres after 3 h, respectively. According to the comparative analysis of the Li2S6 solution and the blank one in Figure 3b, the former has two additional peaks at 218 and 478 cm−1, which are characteristic of Li2S6. The Raman spectra of the Li2S6 solution with added Sup P are similar to those of the Li2S6 solution, which still have two peaks at 218 and 478 cm−1. In the spectra of the Li2S6 solution with Co3O4 spheres, the peaks of Li2S6 are weaker than

nanosheets becoming nanoparticles with a diameter of 20−30 nm. This can be clearly seen in the transmission electron microscopy (TEM) images in Figure S3. The TEM images also show that both the Co3O4 and Co4N spheres are porous structures. The high crystallinity of Co4 N spheres is corroborated by the high-resolution TEM (HRTEM) image in Figure 1d. The fringe spacing of 0.203 nm corresponds to the (111) spacing (0.205 nm) of Co4N.43 The pore size distribution and specific surface area of the Co3O4 and Co4N spheres were in the N2 adsorption− desorption isotherms. As shown in Figure 2, both samples reflect quite similar N2 sorption isotherms and pore size distribution curves. The Brunauer−Emmett−Teller specific surface areas of Co3O4 and Co4N spheres are 51.2 and 48.4 m2 g−1, and the pore volumes are 0.254 and 0.237 cm3 g−1, respectively. The specific surface area and pore volume of Co3O4 spheres are slightly higher than those of Co4N spheres. The high specific surface area and large pore volume can provide abundant pore structure to stockpile sulfur and more adsorption and catalytic sites for polysulfide, thus significantly improving the specific capacity and cycling stability for the lithium−sulfur battery. Many metal oxides and metal nitrides were reported to have strong adsorption for polysulfide and “trap” the lithium polysulfides in the cathodes instead of shuttling to the anode. This will reduce the “shuttle effect” and improve the cycle 6033

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Figure 4. (a) Charge and discharge voltage profiles for the first cycle at current densities of 0.1, 0.2, 0.5, 1, 2, and 5 C. (b) Rate capability of Co4N/70S at different current rates. (c, d) Charge and discharge capacity and Coulombic efficiency versus cycle number at current densities of 0.1 C, 0.5 C, and 1 C. (e) Charge and discharge capacity versus cycle number at current densities of 2 and 5 C.

that of Sup P, but the peak at 218 cm−1 still exists. In the spectra of that with Co4N spheres, there are no obvious peaks appearing at 218 or 478 cm−1. That is because Li2S6 in the solution is almost completely adsorbed by the Co4N spheres and there is no Li2S6 species in the solution, indicating that Co4N spheres have a high affinity for LiPSs. In terms of kinetics, not only the affinity but also the trapping speed, which has not been discussed yet, is crucial for sulfur redox processes. So the trapping speed was also investigated here. Figure 3c and d show the visual discrimination by taking Li2S6 and Raman spectroscopy after 12 h. Both the Li2S6 solutions with Co3O4 spheres and Co4N spheres became colorless; the peak at 218 cm−1 in the Raman spectra of the Li2S6 solution with Co3O4 spheres disappeared. This proved Co3O4 spheres have stronger affinity for polysulfide than carbon materials, but compared with Co4N spheres, the Co4N spheres showed a higher adsorption rate for LiPSs. The stronger and faster adsorption for LiPSs may make Co4N spheres a promising host material of S cathodes for Li−S batteries. Sulfur was incorporated into the Co4N spheres by melt diffusion under 155 °C. Thermogravimetric analysis (TGA) in Figure S4 shows that all of the sulfur is retained in the Co4N/S sample after melting diffusion treatment, yielding a 72.32 wt % composite. Figure S6 shows the characterization of the Co4N/ 70S sample. The XRD results shown in Figure S6 prove the presence of sulfur in the sample. As the scanning electron microscopy (SEM) images in Figure S6b and c illustrate, the surface of Co4N/70S become smoother than that of Co4N spheres, and there are a few additional sulfur particles in the sample, most of the sulfur is distributed in the framework of Co4N spheres. In order to further confirm the distribution pattern of S in the sample, the SEM corresponding element mapping of Co, N, and S are revealed in Figure S5d−g. The elemental distribution maps on a single Co4N/70S spheres clearly reveal the distribution of elemental S is on the Co4N spheres surface. To evaluate the electrochemical properties of the Co4N/70S composites with 72.3 wt % sulfur, the cell was cycled from 1.7 to 2.7 V versus Li/Li+ (Figure 4). The galvanostatic discharge

and charge curves of the first cycle at 0.1 C show that such composites could deliver a very high specific discharge capacity of 1659 mAh g−1 at this rate, which is very close to the theoretical capacity of sulfur (1675 mAh g−1). The Co4N/70S shows an excellent rate performance and high specific capacity as shown in Figure 4a. Under the current densities of 0.1 C, 0.2 C, 0.5 C, 1 C, 2 C, and 5 C, the Co4N/70S shows first-cycle discharge specific capacities of 1659, 1441, 1342, 1280, 1213, and 1144 mAh g−1, respectively. Even when the current density is as high as 8.4 A g−1 (5 C), the first-cycle discharge specific capacity can still maintain 1144 mAh g−1, equivalent to 68.3% of the theoretical capacity of sulfur. Figure 4b shows the rate properties of a Co4N/70S electrode at different current rates. When the current densities increase from 0.1 C to 0.2 C, 0.5 C, 1 C, and 2 C, the reversible capacities are 1657, 1299, 1141, 1010, and 882 mAh g−1, respectively. After the high rate cycling, the current density returns to 0.1 C. An extremely high specific discharge capacity of 1334 mAh g−1 is recovered, which is close to that in the beginning. Besides the good rate performance and high specific capacity, the Co4N/70S also shows excellent cycling stability at various rates. When the current density was 0.1 C, the discharge capacity was above 1300 mAh g−1 after 20 cycles. The charge/ discharge capacity and Coulombic efficiency versus cycle number at a current density of 0.5 and 1 C are shown in Figure 4d. At the current density of 0.5 C, the Coulombic efficiency of the first cycle was above 99.5% and finally remains at nearly 100%. Co4N/70S delivered a discharge capacity above 1100 mAh g−1 at 0.5 C and above 1000 mAh g−1 at 1 C after 100 cycles. After 500 cycles, the discharge capacity can still stabilize at about 930 mAh g−1 at 1 C (as shown in Figure S6). Figure 4e shows an excellent cycling stability at high current densities of 2 and 5 C, and the discharge capacity finally stabilized at 805 and 585 mAh g−1 after 300 cycles, respectively. Even when the cycle number was increased to 1000, the capacity can still stay at 761 and 494 mAh g−1, respectively. The discharge capacity, rate capability, and cycling stability of Co4N/S are very rare in the current studies of Li−S batteries (as shown in Tables S1 and S2). 6034

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Figure 5. First charge and discharge voltage profiles of Co4N/S, Co3O4/S, Co/S, and Sup P/S electrodes at 0.1 C (a) and 1 C (b). (c) Co 2p3/2 X-ray photoelectron spectroscopy of the Co4N phase and Co4N/Li2S6, respectively. (d) First CV Co4N/S, Co3O4/S, Co/S, and Sup P/S at a scan rate of 0.05 mV s −1. (e) Discharge capacity of Co4N/S, Co3O4/S, and Co/S versus cycle number at a current density of 1 C.

study the chemical composition in Co4N, X-ray photoelectron spectroscopy (XPS) was used. Figure 5c is the XPS of Co 2p3/2 in Co4N and Co4N/Li2S6, respectively. The spectrum showed that Co 2p3/2 in Co4N can be divided into two characteristic peaks located at 780.3 and 778.3 eV, respectively. The peak located at 778.3 eV corresponds to metallic cobalt, confirming the existence of cobalt with 0 valency, and the peak at 780.3 eV could be assigned to the Co−N bond. After adsorbing Li2S6, the Co 2p3/2 spectrum has an additional peak at 778.9 eV, indicative of a Co−S bond,44 and the intensity of the former peaks recedes, especially the peak around 778.5 eV. This suggests that cobalt with low valency in Co4N has a strong chemical affinity for sulfur in the LiPSs. Figure 5d shows the first-cyclic voltammetric curve of the Co4N/S, Co3O4/S, and Sup P/S cell in the range of 1.7−2.7 V at a scan rate of 0.05 mV s−1. All the samples show similar charge and discharge profiles: two characteristic reduction peaks observed are attributed to the reduction of the cyclo-S8 to long-chain Li2Sn, then formation of Li2S2 and Li2S, respectively. On forward scan, a broad oxidation peak at about 2.5 V is attributed to the conversion of short-chain to long-chain LiPSs. The CV of Co4N/S is compared with that of Sup P/S; there is nearly no obvious shift in reduction peak, but the distinguishable negative shift in the oxidation peak indicates that the Co4N sphere can facilitate the oxidation of short-chain to long-chain LiPS, thus improving the S utilization and increasing the capacity of the charge process. There is almost no obvious shift in oxidation peak compared with Co/S and Sup P/S. This result indicates better redox reactions taking place at the Co4N electrode. Like the first discharge and charge voltage profiles shown in Figure 5b, the same situation can be seen: the oxidation plateau of the Co4N/S electrode is much lower than that of the Sup P/S electrode. It can be attributed to a Co4N sphere that has a stronger adsorption ability than LiPSs, which catalyzes the reactions from short-chain to long-chain LiPSs

In order to further study the effects of Co4N for lithium− sulfur batteries, we compared the first-cycle charge and discharge processes of the Co4N/S, Co3O4/S, Co/S, and Sup P/S. Figure S4 shows the TGA curves of these composites, and the sulfur content of these composites was about 73%. Figure 5a and b are the first charge and discharge voltage profiles for the first cycle at 0.1 and 1 C for the four composites, respectively. Each of the composites shows a typical voltage and capacity profile for Li−S batteries. There are two voltage plateaus in the discharge process. The first plateau at 2.35 V is attributed to the reduction of cyclo-S8 to solvable long-chain Li2Sn (4 ≤ n ≤ 8), and the second plateau at 2.1 V corresponds to the further reduction of the high-order Li2S4 to Li2S2 and eventually to Li2S. The first process of discharge occurs relatively fast, and the four composites had a similar discharge capacity in this process. But at the second plateau, they exhibit distinct differences. The first-cycle discharge capacities of Co4N/S, Co3O4/S, Co/S, and Sup P/S at 0.1 C were 1657, 1292, 1584, and 1405 mAh g−1, respectively. The discharge capacity of Co4N/S is close to the theoretical capacity of S, and Co/S also shows a rather high capacity of the first cycle at 0.1 C. They are all much higher than the capacity of Sup P/S and Co3O4/S, and at high current densities such as 1 C, this phenomenon is more distinct. So it can be inferred that in comparison with carbon materials Co4N spheres could more effectively improve the initial discharge capacity of the Li−S battery. What is more, this significant improvement mainly depends on the promotion of the capacity of the lower plateau, which suggests that Co4N spheres can catalyze the process of reduction of Li2S4 to Li2S2 and eventually to Li2S. The Co element of low valence in Co4N plays a major role in this catalytic process. This may be attributed to the valence state of the Co ions in Co4N not at the full oxidation valence of +3; they are highly capable of donating electrons, which leads to a strong affinity for sulfur atoms/ions in the LiPSs. In order to 6035

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ACS Nano and then to S8. It was reported in our previous work that one electron lone pair of the N atom can serve as a conductive Lewis base “catalyst” matrix to enhance the adsorption energy of Li in Li2Sn (n = 4−8), which facilitates the oxidization of LiPSs.9,15,31 The fast oxidation process could lead to a better charge capacity, then an improved Coulombic efficiency. The charge capacity of the Co4N/S cell at 0.1 C was 1655 mAh g−1, nearly the theoretical capacity of S, and the Coulombic efficiency of the first cycle was as high as 99.9%. On the other hand, the charge capacity of Co/S and Sup P/S was 1470 and 1315 mAh g−1, respectively. The Coulombic efficiency was only 92.8% and 93.6%. The Co and N in Co4N have strong adsorption capacity for both S and Li in LiPSs, respectively. So the bifunctional catalytic activities of Co4N spheres can increase both the discharge and charge capacity for the lithium−sulfur battery and improve the Coulombic efficiency. The fast and strong affinity of Co4N spheres for polysulfide can reduce the “shuttle effect” during the discharge/charge process, so the Co4N/S electrode exhibits high capacity, good rate performance, and excellent cycling stability. Compared with the discharge capacity of Co3O4/S and Co/S composites under the current density of 1 C, as shown in Figure 5e, the capacity of Co4N/S is about 1000 mAh g−1, and there is nearly no obvious capacity fading from 4 to 100 cycles. Because the Co3O4 spheres have a relatively strong affinity for polysulfide in comparison with carbon materials, it shows a satisfactory cycling stability for the Li−S battery. But the capacity is much lower than Co4N/S; it stabilizes at about 650 mAh g−1. The Co/S composite shows a higher discharge capacity than Co3O4/S at first, but the weak Coulombic efficiency leads to worse cycling stability; the capacity fell fast, and after 100 cycles there was only less than 600 mAh g−1 left. Figure S7 shows that the rate performance of the Co4N/S cathode is far superior than those of the other composite cathodes, and the XPS of the S element in the metal lithium anode and electrolyte after 100 cycles (Figures S8 and S9) shows that Co4N can efficiently reduce the “shuttle effect” during the discharge and charge process. The Co4N spheres not only have stronger affinity for polysulfide than Co3O4 but can also adsorb more polysulfide than Co3O4. Figure 6a and b show the visual discrimination by comparing 5, 10, 20, and 30 mg of Co4N spheres and Co3O4

spheres, respectively, added to the Li2S6 solution. After 12 h, the Li2S6 solutions containing Co4N spheres became almost colorless, but only the solution with 30 mg of Co3O4 spheres became colorless. The color of the other solutions only became lighter, indicating that there were still leftover Li2S6 species in the solution. The excellent adsorbing capacity of Co4N spheres for polysulfide may lead to good electrochemical performance for the Li−S battery even at a high loading of insulating sulfur. When the weight ratio of elemental S and Co4N spheres was set to 9:1, the TGA curve showed the S content in this sample is 89.64 wt % (Figure S10, named Co4N/90S). However, as was expected, the Co4N/90S electrode also exhibited an outstanding electrochemical property (as shown in Figure 6). At the rate of 0.1 C, the initial discharge capacity of the Co4N/90S electrode could achieve 1428 mAh g−1. When the rate increases to 2C, the Co4N/90S electrode still can release a high discharge specific capacity of 720 mAh g−1, and this is an excellent result compared to recent reports. In addition to a good rate performance, the Co4N/90S cathode also shows excellent cycling stability and high specific capacity at various rates. At a current rate of 0.5 C, the Co4N/90S cathode could deliver a high initial discharge capacity of 1201 mAh g−1, and this capacity still remained at 920 mAh g−1 after 100 cycles. At a high rate of 1 C, after 100 cycles, the reversible specific capacity was as high as 801 mAh g−1. Even when the current densities increase to 2 and 5 C, there was still a discharge capacity finally stabilizing at 670 and 520 mAh g−1 after 300 cycles. Even when the cycle number was increased to 800, the capacity can still remain at 690 and 440 mAh g−1, respectively. Compared with the discharge capacity of the Co3O4/90S composite (the same melt-diffusion process as Co4N/90S) under a current density of 1C (when the sulfur content was 70 wt %, Co3O4/S shows a satisfactory cycling stability), when the sulfur content was increased to 90 wt %, the cycling stability of Co3O4/S became worse, the capacity decreased rapidly, and after 100 cycles only about 450 mAh g−1 remained. This indicates the hollow Co4N mesoporous sphere is conducive to immobilize sulfur and reduce the dissolution of polysulfide at ultrahigh sulfur content. Even after increasing the weight ratio of elemental sulfur and Co4N spheres to 19:1 (sulfur content is 94.88%, named Co4N/ 95S), the Co4N/S cathode still exhibited excellent electrochemical properties. As shown in Figure 5, the Co4N/95S cathode could exhibit an extremely high specific discharge capacity of 1259 mAh g−1 at 0.1 C. After 100 cycles at 1 C, the Co4N/S cathode also maintained a high specific capacity of 640 mAh g−1, and after 800 cycles at 2 C, the discharge capacity still remained at 540 mAh g−1. Compared with the Co3O4/90S cathode, there is a great improvement in the specific capacity and cycling stability, and this is a very high level at present (Table S3).

CONCLUSIONS In summary, Co4N mesoporous spheres present a uniform distribution, and the displayed porous structure has been successfully synthesized via an easy and convenient method. With high affinity, speedy trapping, high absorbing capacity for polysulfides, and the bifunctional catalyzing for sulfur redox processes, this Co4N sphere has proven to be an ideal matrix for sulfur-active materials. When the Co4N sphere composed with 72.32 wt % sulfur is evaluated as a cathode material for lithium−sulfur batteries, it exhibited a large specific discharge capacity of 1659 mAh g−1 at 0.1 C, high Coulombic efficiency, good rate capability, and excellent cycling performance with a

Figure 6. Sealed vials of a lithium polysulfide solution (Li2S6 dissolved in DOL/DME solvents) containing 5, 10, 20, and 30 mg of Co4N phase (a) and Co3O4 phase (b) after 12 h, respectively. 6036

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Figure 7. (a) Rate capability of Co4N/90S and Co4N/95S at different current rates. (b) Discharge capacity of Co4N/90S at 0.5 C and Co4N/ 90S, Co4N/95S, and Co3O4/90S versus cycle number at a current density of 1 C. (c) Charge and discharge capacity of Co4N/90S versus cycle number at current densities of 2 and 5 C and Co4N/95S at 2 C.

reversible capacity of 1000 mAh g−1 at 1 C after 100 cycles. Even at increased 9:1 and 19:1 weight ratios of elemental sulfur and Co4N spheres (90% and 95 wt % S loadings), the discharge capacity of the first cycle is still as high as 1428 and 1259 mAh g−1, respectively. The reversible capacity at 2 C after 800 cycles was above 690 and 540 mAh g−1, respectively. The facile approach and its results could be used to guide the realization of S-based high energy density batteries.

30 mg amount of Sup P, mesoporous Co4N, and Co3O4 spheres were then added to there different vials, respectively. All procedures were completed in an Ar-filled glovebox. Characterization of the Materials. X-ray diffraction (XRD) patterns of the samples were recorded on a Philips Analytical X-pert diffractometer with Cu Kα radiation (λ = 0.1548 nm) at 40 kV and 30 mA and a step of 0.02. Data were recorded ranging from 20 to 80 degrees. SEM was performed on a Hitachi S-4800 scanning electron microscope at an accelerating voltage of 15 kV. TEM observations were carried out on FEI Tecnai F30 microscopes at 300 kV. Thermogravimetric analysis was carried out on a PerkinElmer instrument. Nitrogen adsorption and desorption isotherms at 77 K were characterized by a Micromeritics Tristar 3020 analyzer surface area and pore-size analyzer. Raman spectra were performed using an XploR Raman microscope with an excitation wavelength of 785 nm. Electrochemical Measurements. To evaluate the electrochemical properties of the Co4N/S composite as a cathode material, its electrochemical testing was conducted. The cathode electrode consists of 70 wt % active material (sulfur composites), 20 wt % Super P, and 10 wt % binder (water-soluble 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 1.5−2 mg cm−2 (70 wt % S loading) and 2.4−2.8 mg cm−2 (90 wt % S loading), and 40 μL of electrolyte was added into the CR2016-type coin cells. These cells were cycled on a NEWARE BTS-5 V/20 mA type battery charger (Shenzhen, China) testing system at room temperature, and 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 were 1 C. The cell was activated at a 0.1 C rate for 3 cycles when the current densities were 1 C, 2 C, and 5 C.

EXPERIMENTAL SECTION Synthesis of Co3O4 Spheres. All chemicals were analytical grade and used without further purification. Two grams of Co(CH3COO)2 and 0.2 g of polyvinylpyrrolidone (Mw = 18 000 g mol−1) were dissolved into 160 mL of ethylene glycol. After stirring for 3 h, the mixture was then transferred into an autoclave and maintained at 200 °C for 12 h. Then the reactant was cooled to room temperature. The precipitate was collected by centrifugation, then washed with ethanol and distilled water several times, subsequently dried at 50 °C, then calcined at 550 °C in a temperature-programmed muffle furnace for 4 h in air to get the Co3O4 spheres. Synthesis of Co4N Spheres. The as-prepared Co3O4 spheres were heated to 400 °C in an ammonia atmosphere for 4 h with a slow heating ramp (room temperature to 300 °C, 5 °C min−1; 300 to 400 °C, 1 °C min−1). After being cooled to room temperature, mesoporous Co4N spheres were thus obtained. Synthesis of Co Nanoparticles. The Co3O4 spheres were heated to 700 °C in an ammonia atmosphere for 4 h with a slow heating ramp (room temperature to 600 °C, 5 °C min−1; 600 to 700 °C, 1 °C min−1). After being cooled to room temperature, Co nanoparticles were thus obtained. Preparation of the Sulfur Composites. The Co4N/S composite was prepared via the classical melt-diffusion method. First, the required amount of elemental sulfur and mesoporous Co4N spheres (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 Co3O4/S, Co/S, and Sup P/S composite was prepared by the same method as the Co4N/S one. Polysulfide Adsorption Sample Preparation. The 5 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. A

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b01945. Additional XRD, TEM, XPS, EDX, TG, and electrochemical performance (PDF) 6037

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AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Quan-Feng Dong: 0000-0002-4886-3361 Notes

The authors declare no competing financial interest.

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