3D Hollow rGO Microsphere Decorated with ZnO Nanoparticles as Efficient Sulfur Host for High-Performance Li-S Battery

Lithium-sulfur battery (LSB) will become the next generation energy storage device if its severe shuttle effect and sluggish redox kinetics can be effectively addressed. Here, a unique three-dimensional hollow reduced graphene oxide microsphere decorated with ZnO nanoparticles (3D-ZnO/rGO) is synthesized to decrease the dissolution of lithium polysulfide (LiPS) into the electrolyte. The chemical adsorption of ZnO on LiPS is combined with the physical adsorption of 3D-rGO microsphere to synergistically suppress the shuttle effect. The obtained 3D-ZnO/rGO can provide sufficient space for sulfur storage, and effectively alleviate the repeated volume changes of sulfur during the cycle. When the prepared S-3D-ZnO/rGO was used as the cathode in LSB, an initial discharge specific capacity of 1277 mAh g−1 was achieved at 0.1 C. After 100 cycles, 949 mAh g−1 can still be maintained. Even at 1 C, a reversible discharge specific capacity of 726 mAh g−1 was delivered.


Introduction
Lithium-ion battery (LIB) is the most widely used rechargeable battery, with many advantages: high energy density, long service life and low cost [1][2][3]. After development for more than 20 years, the specific capacity of the commercial cathode material is close to its theoretical value, which still cannot meet the growing energy need. Therefore, the search for a new rechargeable battery with high energy density has become urgent [4,5]. Recently, a lithium-sulfur battery (LSB) has caught the attention of researchers all over the world. Elemental sulfur has a high theoretical specific capacity of 1675 mAh g −1 , indicating its great potential as energy storage material. The low working voltage of LSB (~2.2 V) can adapt to the commercial need. In addition, sulfur also has the advantage of being a sufficient resource, having low cost, and being harmless to the environment [6][7][8][9][10][11]. Therefore, LSB is regarded as the most potential substitute for commercial LIB, but the performance of LSB is still difficult to reach the current level of commercial LIB, for these reasons: (1) the low conductivity of elemental sulfur and its discharge products (Li 2 S 2 and Li 2 S). (2) The dissolution of the reaction intermediate (Li 2 S n , 4 ≤ n ≤ 8) into the electrolyte. (3) The repeated volume changes of elemental sulfur during the cycling process [12][13][14].
In recent years, researchers have conducted a lot of work on the modification of cathode materials for the improved performance of LSB. The use of the carbon-sulfur composite cathode is considered to be an effective way to realize the improvement [15,16]. Among the carbon materials, graphene has received extensive attention. Graphene has many advantages, such as high specific surface area, excellent electrical conductivity, high structural stability, high strength, high flexibility, and easy The S-3D-ZnO/rGO, super P (Guotai Huarong Chemical New Material Co., Ltd. China) and polyvinylidene fluoride (PVDF, Aladdin Reagent Co., Ltd. Shanghai, China) were mixed according to a mass ratio of 8:1:1. N-methylpyrrolidone (NMP, Aladdin Reagent Co., Ltd. Shanghai, China) was used to form a uniform slurry with a certain viscosity. The slurry was coated on the carbon-containing aluminum foil by a doctor blade. After being dried, the aluminum foil was cut into disks with diameters of 10 mm, and the disks were pressed at a pressure of 10 MPa for 1 min. The weight of the active material content on the disks was weighed using a balance. The sulfur content per unit area is between 1.27 and 1.88 mg cm −2 . The prepared disk was used as the cathode, the lithium foil (Qinhuangdao Lithium Co., Ltd., Qinhuangdao, China) was used as the counter electrode, the Celgard 2400 film (Shenzhen Weifeng Electronics Co., Ltd., Shenzhen, China) was used as a separator. Moreover, 1 M LiTFSI in 1,2-dimethoxy ethane (DME) and 1,3-dioxolane (DOL) (1:1 v/v) with 0.1 M LiNO3 were used as the electrolyte (Guotai Huarong Chemical New Material Co., Ltd., Zhangjiagang, China). The assembly of the button cell (CR2032) was conducted in the Ar-filled glove box. The Neware battery tester (BTS 4000, Neware Inc., Shenzhen, China) was used to evaluate the cycling stability and rate performance. The electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were explored by electrochemical workstation (Princeton Applied Research, Versa STAT3, Ametek, PA, USA).

Theoretical Calculations
The Vienna ab initio simulation package (VASP) was employed to conduct the DFT calculation, and the results were visualized in Materials Studio. A plane-wave cutoff of 400 eV was set. The Li2S6 adhesion bonding energy (Eb) was calculated as follows: Eb = ELi2S6/ZnO -ELi2S6 -EZnO, where ELi2S6/ZnO, ELi2S6, and EZnO represent the total energy of the system, isolated Li2S6 and ZnO, respectively.

Results
The ZnO/GO composite was prepared by the sol-gel method at first ( Figure 1). Subsequently, the ZnO/GO sheets were bent and stacked under the action of spray drying to form a hollow sphere structure, which is named 3D-ZnO/GO. In order to further improve the conductivity of ZnO/GO, high temperature reduction is carried out to remove a large number of oxygen-containing groups on GO, resulting in 3D-ZnO/rGO. Finally, 3D-ZnO/rGO is composited with S by simple melt diffusion method to synthesize S-3D-ZnO/rGO. For comparison, pure ZnO is prepared by the sol-gel method. As shown in Figure 2a, the agglomeration in pure ZnO is serious, resulting in low specific surface area and low adsorption For comparison, pure ZnO is prepared by the sol-gel method. As shown in Figure 2a, the agglomeration in pure ZnO is serious, resulting in low specific surface area and low adsorption capacity for sulfur and LiPS. The SEM image of the flaky ZnO/rGO is performed in Figure 2b. The graphene sheets are stacked on top of each other, and a large amount of ZnO particles are evenly distributed on graphene. The graphene can provide a large number of active sites for heterogeneous nucleation, and the growth of ZnO nanoparticles and the uniform dispersion of ZnO nanoparticles are achieved. At the same time, graphene can enhance the electron conductivity, and its good flexibility can improve the structural stability of the cathode. However, the two-dimensional structure formed by the simple stack reduces the effective reaction area, resulting in a decrease in the effective utilization of ZnO. The SEM images of 3D-ZnO/rGO are performed in Figure 2c,d, the originally pleated graphene is crimped into spheres by spray drying, and their diameters are distributed in the range of 2-5 µm. The surface of the microsphere is evenly distributed with a large amount of ZnO nanoparticles. It should be pointed out that the hollow structure can achieve the high sulfur loading and reduce the volume change of the sulfur through the internal space of the hollow spherical structure. The EDS mappings in Figures S4 and S5 further confirm the uniform distribution of carbon, oxygen, and zinc in 3D-ZnO/rGO and ZnO/rGO. Nanomaterials 2020, 10, x FOR PEER REVIEW 4 of 13 capacity for sulfur and LiPS. The SEM image of the flaky ZnO/rGO is performed in Figure 2b. The graphene sheets are stacked on top of each other, and a large amount of ZnO particles are evenly distributed on graphene. The graphene can provide a large number of active sites for heterogeneous nucleation, and the growth of ZnO nanoparticles and the uniform dispersion of ZnO nanoparticles are achieved. At the same time, graphene can enhance the electron conductivity, and its good flexibility can improve the structural stability of the cathode. However, the two-dimensional structure formed by the simple stack reduces the effective reaction area, resulting in a decrease in the effective utilization of ZnO. The SEM images of 3D-ZnO/rGO are performed in Figure 2c,d, the originally pleated graphene is crimped into spheres by spray drying, and their diameters are distributed in the range of 2-5 μm. The surface of the microsphere is evenly distributed with a large amount of ZnO nanoparticles. It should be pointed out that the hollow structure can achieve the high sulfur loading and reduce the volume change of the sulfur through the internal space of the hollow spherical structure. The EDS mappings in Figures S4 and S5 further confirm the uniform distribution of carbon, oxygen, and zinc in 3D-ZnO/rGO and ZnO/rGO. The TEM image of the 3D-ZnO/rGO is displayed in Figure 3a. The hollow structure is obviously observed, and ZnO nanoparticles are densely distributed on the surface of microsphere. The TEM image obtained by magnifying the edge of the microsphere is shown in Figure 3b. ZnO nanoparticles are explored more clearly with the size of 20-60 nm. The small amount of large diameter ZnO particles may be due to the fact that the graphene dispersion contains some water and the ZnO is hydrophilic, so the ZnO particles synthesized during the reaction tend to agglomerate to form large ZnO particles. It can be seen from Figure 3c and Figure S1 that the ZnO particles are composed of many fine ZnO crystal grains, wherein the lattice spacing of 0.281 and 0.249 nm corresponds to the The TEM image of the 3D-ZnO/rGO is displayed in Figure 3a. The hollow structure is obviously observed, and ZnO nanoparticles are densely distributed on the surface of microsphere. The TEM image obtained by magnifying the edge of the microsphere is shown in Figure 3b. ZnO nanoparticles are explored more clearly with the size of 20-60 nm. The small amount of large diameter ZnO particles may be due to the fact that the graphene dispersion contains some water and the ZnO is hydrophilic, so the ZnO particles synthesized during the reaction tend to agglomerate to form large ZnO particles.
It can be seen from Figure 3c and Figure S1 that the ZnO particles are composed of many fine ZnO crystal grains, wherein the lattice spacing of 0.281 and 0.249 nm corresponds to the (100) and (002) crystal faces, respectively. The SAED pattern of ZnO shown in Figure 3d shows a plurality of diffraction rings, indicating the polycrystalline properties of the ZnO particles. Nanomaterials 2020, 10, x FOR PEER REVIEW 5 of 13 (100) and (002) crystal faces, respectively. The SAED pattern of ZnO shown in Figure 3d shows a plurality of diffraction rings, indicating the polycrystalline properties of the ZnO particles. The SEM results of S-ZnO/rGO are shown in Figure 4a,b, the three-dimensional fold structure formed by the interweaving of graphene sheets was well preserved, and no obvious sulfur blocks were observed, indicating the uniform sulfur loading, which is also performed in Figure 4c. Similarly, after sulfur loading, spherical morphology and distinct graphene sheet fold structure are still maintained (Figure 4d,e). Compared with the 3D-ZnO/rGO microspheres, ZnO nanoparticles are difficult to observe, indicating that the elemental sulfur in the molten state diffuses evenly during the hydrothermal process. The element distribution of S is characterized on the microsphere, which can prove that S is uniformly distributed (Figure 4f). Furthermore, element distributions of Zn and C prove that the ZnO particles remain uniform on the graphene surface after hydrothermal reaction. The SEM results of S-ZnO/rGO are shown in Figure 4a,b, the three-dimensional fold structure formed by the interweaving of graphene sheets was well preserved, and no obvious sulfur blocks were observed, indicating the uniform sulfur loading, which is also performed in Figure 4c. Similarly, after sulfur loading, spherical morphology and distinct graphene sheet fold structure are still maintained (Figure 4d,e). Compared with the 3D-ZnO/rGO microspheres, ZnO nanoparticles are difficult to observe, indicating that the elemental sulfur in the molten state diffuses evenly during the hydrothermal process. The element distribution of S is characterized on the microsphere, which can prove that S is uniformly distributed (Figure 4f). Furthermore, element distributions of Zn and C prove that the ZnO particles remain uniform on the graphene surface after hydrothermal reaction.      Raman spectra are shown in Figure 6a. It can be seen that all three samples have a weak peak at about 430 cm −1 , which corresponds to the ZnO crystal. Two distinct broad peaks at around 1344 and 1580 cm −1 are attributed to the D peak and G peak of graphene. The calculation results show that the I D /I G values of ZnO/rGO and 3D-ZnO/rGO composites are both 1.03, which is significantly higher than the I D /I G value (0.95) of ZnO/GO composites. The result shows that GO can be effectively reduced by high-temperature. After sulfur loading, two additional peaks at 217 and 472 cm −1 are detected in S-ZnO/rGO and S-3D-ZnO/rGO (Figure 6b), which is due to the S-S bond vibration of elemental sulfur [32]. At the same time, the calculation shows that the I D /I G values of S-ZnO/rGO and S-3D-ZnO/rGO composites are 1.08 and 1.10, respectively, which shows a little increase. The increase shows that the degree of disorder of the obtained rGO is promoted, which may be related to the bonding between graphene and sulfur. Raman spectra are shown in Figure 6a. It can be seen that all three samples have a weak peak at about 430 cm −1 , which corresponds to the ZnO crystal. Two distinct broad peaks at around 1344 and 1580 cm −1 are attributed to the D peak and G peak of graphene. The calculation results show that the ID/IG values of ZnO/rGO and 3D-ZnO/rGO composites are both 1.03, which is significantly higher than the ID/IG value (0.95) of ZnO/GO composites. The result shows that GO can be effectively reduced by high-temperature. After sulfur loading, two additional peaks at 217 and 472 cm −1 are detected in S-ZnO/rGO and S-3D-ZnO/rGO (Figure 6b), which is due to the S-S bond vibration of elemental sulfur [32]. At the same time, the calculation shows that the ID/IG values of S-ZnO/rGO and S-3D-ZnO/rGO composites are 1.08 and 1.10, respectively, which shows a little increase. The increase shows that the degree of disorder of the obtained rGO is promoted, which may be related to the bonding between graphene and sulfur.   Raman spectra are shown in Figure 6a. It can be seen that all three samples have a weak peak at about 430 cm −1 , which corresponds to the ZnO crystal. Two distinct broad peaks at around 1344 and 1580 cm −1 are attributed to the D peak and G peak of graphene. The calculation results show that the ID/IG values of ZnO/rGO and 3D-ZnO/rGO composites are both 1.03, which is significantly higher than the ID/IG value (0.95) of ZnO/GO composites. The result shows that GO can be effectively reduced by high-temperature. After sulfur loading, two additional peaks at 217 and 472 cm −1 are detected in S-ZnO/rGO and S-3D-ZnO/rGO (Figure 6b), which is due to the S-S bond vibration of elemental sulfur [32]. At the same time, the calculation shows that the ID/IG values of S-ZnO/rGO and S-3D-ZnO/rGO composites are 1.08 and 1.10, respectively, which shows a little increase. The increase shows that the degree of disorder of the obtained rGO is promoted, which may be related to the bonding between graphene and sulfur.   An XPS test was conducted to study the element valence of S-3D-ZnO/rGO composites. The characteristic peaks of C, Zn, O and S are performed in the full spectrum (Figure 8a). The S 2p spectrum (Figure 8b) can be decomposed into four equivalent small peaks, at the binding energies of 162.1, 163.6, 164.8, and 168.8 eV, corresponding to the Zn-S, S 2p 3/2 , S 2p 1/2 , and sulfate bonds, respectively. In Figure 8c, the peaks at 1022.7 and 1045.7 eV correspond to Zn 2p 3/2 and Zn 2p 1/2 , respectively. The spectrum of C 1s can be decomposed into five equivalent small peaks (Figure 8d). The equivalent substitution peaks at 284.6 and 284.8 eV correspond to the C=C bond and the C-C bond in the sp 2 hybrid structure, respectively. The equivalent substitution peaks at 286.6 and 289.5 eV are attributed to C-O, C=O and O=C-O bonds, respectively, indicating that the obtained rGO after high temperature reduction still retains a certain number of oxygen-containing functional groups. The peak at 285.6 eV is due to the C-S bond, indicating a chemical bond between elemental sulfur and graphene. XPS results show that the sulfur in the S-3D-ZnO/rGO composite is mainly in the form of sulfur molecules, and a small amount of sulfur combines with ZnO or graphene to form specific chemical bonds. The formation of chemical bonds can enhance the combination of sulfur and ZnO/rGO composite, thus effectively improving the adsorption and immobilization of ZnO/rGO composite on active sulfur and polysulfide during the cycling process. An XPS test was conducted to study the element valence of S-3D-ZnO/rGO composites. The characteristic peaks of C, Zn, O and S are performed in the full spectrum (Figure 8a). The S 2p spectrum (Figure 8b) can be decomposed into four equivalent small peaks, at the binding energies of 162.1, 163.6, 164.8, and 168.8 eV, corresponding to the Zn-S, S 2p3/2, S 2p1/2, and sulfate bonds, respectively. In Figure 8c, the peaks at 1022.7 and 1045.7 eV correspond to Zn 2p3/2 and Zn 2p1/2, respectively. The spectrum of C 1s can be decomposed into five equivalent small peaks (Figure 8d). The equivalent substitution peaks at 284.6 and 284.8 eV correspond to the C=C bond and the C-C bond in the sp 2 hybrid structure, respectively. The equivalent substitution peaks at 286.6 and 289.5 eV are attributed to C-O, C=O and O=C-O bonds, respectively, indicating that the obtained rGO after high temperature reduction still retains a certain number of oxygen-containing functional groups. The peak at 285.6 eV is due to the C-S bond, indicating a chemical bond between elemental sulfur and graphene. XPS results show that the sulfur in the S-3D-ZnO/rGO composite is mainly in the form of sulfur molecules, and a small amount of sulfur combines with ZnO or graphene to form specific chemical bonds. The formation of chemical bonds can enhance the combination of sulfur and ZnO/rGO composite, thus effectively improving the adsorption and immobilization of ZnO/rGO composite on active sulfur and polysulfide during the cycling process. The study of the performance of the redox reaction of S-3D-ZnO/rGO composite was complemented with the CV test (Figure 9a). The CV curve shows two reduction peaks at about 2.06 and 2.32 V, corresponding to the reduction of S8 to long-chain polysulfides (Li2Sn, 4 ≤ n ≤ 8) and the following reaction of the transformation from long-chain polysulfides to Li2S2/Li2S. During the subsequent reverse scanning, an oxidation peak at about 2.38 V is observed, which is due to the reverse reaction of the formation of S8 for the delithiation of Li2S and Li2S2. It is worth noting that the The study of the performance of the redox reaction of S-3D-ZnO/rGO composite was complemented with the CV test (Figure 9a). The CV curve shows two reduction peaks at about 2.06 and 2.32 V, corresponding to the reduction of S 8 to long-chain polysulfides (Li 2 S n , 4 ≤ n ≤ 8) and the following reaction of the transformation from long-chain polysulfides to Li 2 S 2 /Li 2 S. During the subsequent reverse scanning, an oxidation peak at about 2.38 V is observed, which is due to the reverse reaction of the formation of S 8 for the delithiation of Li 2 S and Li 2 S 2 . It is worth noting that the CV curves of the first three cycles perform a high coincidence, indicating the good reversibility and stability of the S-3D-ZnO/rGO electrode. seen that the cycle performance of S-3D-ZnO/rGO is more stable. The higher stability can be attributed to the following reasons: (1) the 3D-ZnO/rGO can coat sulfur inside the hollow spherical structure, thereby reducing the dissolution of LiPS into the electrolyte [33][34][35][36]. (2) The internal space of the hollow spherical structure can effectively alleviate the volume change of sulfur during the cycling process, resulting in the high structural integrity of the electrode. We can intuitively observe that 3D-ZnO/rGO has stronger adsorption capacity for polysulfide through the adsorption experiment in Figure S2; secondly, we can also observe the volume change of electrode material after the cycling experiment through SEM after cycling. As shown in Figure S3, at 1 C after 100 cycles, the morphology of 3D-ZnO/rGO is still a hollow sphere, while the morphology of ZnO/rGO has collapsed. The capacity retention rates reached 93.5% and 94.3%, respectively. Compared with S-ZnO/rGO, S-3D-ZnO/rGO cathode shows higher initial discharge specific capacity and capacity retention for effective confinement on LiPS. The carefully introduced three-dimensional hollow structure perfectly encapsulates sulfur and prevents its diffusion into the electrolyte. At the same time, the polarization in S-3D-ZnO/rGO (∆E = 0.21 V) is obviously suppressed compared with S-ZnO/rGO (∆E = 0.28 V), which can be attributed to the enhanced effective utilization rate of ZnO in S-3D-ZnO/rGO. In the two-dimensional layered structure of ZnO/rGO, the graphenes are stacked on each other, such that the effective contact area of ZnO with polysulfide is reduced. The ZnO stacked inside the graphene sheet layer is not effectively utilized, and the chemical adsorption of ZnO cannot be not fully exerted. The three-dimensional hollow spherical structure of 3D-ZnO/rGO avoids the stacking of graphene, and the ZnO is fully exposed, providing sufficient active sites for the reaction of ZnO and LiPS, to produce effective suppressing on the shuttle effect.
Cycling performance of S-3D-ZnO/rGO and S-ZnO/rGO composite cathodes at 0.1 C is performed in Figure 9d. After 100 cycles, the discharge specific capacities of S-ZnO/rGO and S-3D-ZnO/rGO cathodes decreased to 838 and 949 mAh g −1 , and the corresponding capacity retention rates are 67.3% and 74.3%, respectively. Furthermore, the as-developed S-3D-ZnO/rGO cathode can fulfill decent sulfur electrochemistry, even under a high sulfur loading of 4.5 mg cm −2 ( Figure S6). It can be seen that the cycle performance of S-3D-ZnO/rGO is more stable. The higher stability can be attributed to the following reasons: (1) the 3D-ZnO/rGO can coat sulfur inside the hollow spherical structure, thereby reducing the dissolution of LiPS into the electrolyte [33][34][35][36]. (2) The internal space of the hollow spherical structure can effectively alleviate the volume change of sulfur during the cycling process, resulting in the high structural integrity of the electrode. We can intuitively observe that 3D-ZnO/rGO has stronger adsorption capacity for polysulfide through the adsorption experiment in Figure S2; secondly, we can also observe the volume change of electrode material after the cycling experiment through SEM after cycling. As shown in Figure S3, at 1 C after 100 cycles, the morphology of 3D-ZnO/rGO is still a hollow sphere, while the morphology of ZnO/rGO has collapsed.
The same advantage of the S-3D-ZnO/rGO cathode is demonstrated in the rate performance test (Figure 9e). As the current density gradually increases, the discharge specific capacities of both gradually decrease, which is caused by high polarization, low sulfur utilization and limited charge transfer at high current density. At 1 C, the discharge specific capacities of S-ZnO/rGO and S-3D-ZnO/rGO are 654 and 726 mAh g −1 , respectively. When the current density is returned from 1 C to 0.1 C, the discharge capacities of 987 and 1034 mAh g −1 are obtained again. Compared with the S-ZnO/rGO cathode, S-3D-ZnO/rGO exhibits a higher discharge specific capacity under the same test conditions. This is because the spherical structure of 3D-ZnO/rGO can maintain structural integrity even at a high c-rate, resulting in effective suppression of the shuttle effect.
In order to further understand the electrochemical performance of the S-3D-ZnO/rGO composite, the EIS results of S-3D-ZnO/rGO cathode before cycling and at the 10th cycle were tested in the frequency range 100 kHz to 0.01 Hz (Figure 9f). After ten cycles, the charge transfer resistance of the battery was reduced from 27 Ω to 15 Ω. The high resistance before cycling for the cathode material is perhaps not sufficiently contacted with the electrolyte. However, as the cycle progresses, the cathode material is more in contact with the electrolyte, thereby lowering the charge transfer resistance of the battery. In addition, after 10 cycles, the slope of the EIS in the low frequency region increased, indicating that the Warburg impedance in the battery is reduced.
The working mechanism between the ZnO and LiPS was explored by density functional theory (DFT) calculations. The optimized geometrical configuration of Li 2 S 6 -ZnO is shown in Figure 10a. The configuration demonstrated the bond of the S atoms in Li 2 S 6 and the Zn atoms on ZnO (101) surface, which was consistent with the above XPS result. At the same time, a high binding energy of −2.49 eV was achieved, indicating the strong adsorption of ZnO toward LiPS. Apart from the strong adsorption effect, ZnO also demonstrated the ability to boost the sulfur species decomposition. As shown in Figure 10b, a small energy barrier of 0.12 eV for Li 2 S decomposition was performed and the corresponding geometrical configurations were displayed in Figure 10c, which confirmed the remarkable catalysis ability of ZnO on sulfur species transformation.

Conclusions
In summary, an effective sulfur host (3D-ZnO/rGO) is prepared by a simple method. The obtained 3D-ZnO/rGO possesses spherical hollow structure, which is beneficial for the sulfur loading. ZnO can effectively confine LiPS by Zn-S bond and prevent the dissolution of LiPS into the electrolyte. At the same time, the 3D-rGO can immobilize LiPS by physical adsorption. The obvious relief on the volume change can also be realized by the designed structure. When used as a cathode, the polarization is relieved in S-3D-ZnO/rGO electrode, and the excellent cycling stability and outstanding rate performance are achieved. The results show that S-3D-ZnO/rGO is an ideal cathode material for LSB.