Nanoemulsion‐directed assembly of hierarchical ZnS@C nanospheres with penetrating pores for sodium storage

To follow up on the performance of lithium‐ion batteries (LIBs), transition metal sulfides (TMSs) have been developed as promising carbon alternatives for sodium‐ion batteries (SIBs). Although attractive, it is still a great challenge to fulfill their capacity utilization with high cycling performance. Herein, a nanoemulsion‐directed method has been developed to control the spherical arrangement of ZnS@C units with both penetrating macropores from the center to the surface and inner mesopores distributed among the bulks. With respect to ion diffusion, the penetrating macropores could serve as the built‐in ion‐buffer reservoirs to keep a steady flow of electrolyte, while the inner mesopores facilitate the ion diffusion across the whole bulks. In terms of stability, the radical porous structure could work as self‐supported vertical bones to accommodate the volume change from both lateral and vertical sides. Besides, the localized carbon distributed among the ZnS nanoparticles not only acts as binding agents to join the numerous ZnS nanoparticles but also endows the radical bones with effective electron transmission capability. As a proof of concept, such hydrangea‐like ZnS@C nanospheres deliver sodium storage performance with high‐rate and long‐cycling capability. This nanoemulsion‐directed approach is anticipated for other TMSs with penetrating pores for post‐lithium‐ion batteries applications.


| INTRODUCTION
On the way toward the post-lithium-ion batteries (LIBs) era, sodium-ion batteries (SIBs) have gathered both academic and industrial interest due to the similar elemental properties of sodium to lithium but abundant in nature. [1][2][3] Although attractive, SIBs are currently hindered by the lack of suitable host materials to match the larger radius of Na + than Li + with favorable reaction kinetics. To follow up on the performance of LIBs, it is vital to develop functional electrode materials with high theoretical capacity and long-cycle stability. [4][5][6] Notably, transition metal sulfides (TMSs) deliver higher both theoretical gravimetric and volumetric capacity than carbon via conversion or alloying mechanism for Na + storage but are limited by the sluggish kinetics and large volume change during the sodiation/desodiation process. [7][8][9][10] It is believed that both ion transport resistance and distance are related to the kinetics of host materials. To fulfill the role of TMSs in accelerating the maturity of SIBs, great efforts have been devoted to the nanostructure engineering of TMSs into functional porous nanostructures with merits of favorable ion/ electron transfer and breathable structure. 11,12 For example, hollow nanospheres or yolk-shelled nanospheres with multiple layers have been designed as typical electrode materials for Na + storage, with the result of improving capacity and stability to a certain degree. [13][14][15] However, the presence of the hollow cavities contributes to the performance at the expense of volumetric capacity. In addition, the multiple layers are independent of each other without any vertical connection, which is vulnerable to collapse from volume change. Therefore, it is desirable to have a hierarchical spherical arrangement of TMSs with vertical pores radiating out from the center to the surface. In this case, the vertical porous structure not only serves as the self-supported bones to withstand the volume change from both lateral and vertical sides but also acts as three-dimensional (3D) channels to facilitate electron transmission and electrolyte permeation. 16,17 Although attractive, neither the architecture nor the synthetic method of TMSs has been reported minimally in the literature.
To make full use of TMSs as anode materials with high-rate capability, it is also necessary to improve their electronic conductivity. It is believed that the downsized TMS nanocrystals are in favor of shortening ion diffusion distance and enhancing electron transmission; they are vulnerable to aggregation yet. In this respect, integration of TMS-based nanocrystals with conductive species, such as carbon, 18,19 graphene, 20,21 MXene, 22,23 and so on, has been rapidly developed as functional electrode materials for metal-ion batteries. To further develop their structure-activity relationship, the key point locates at the distribution between TMSs and conductive additives. 24,25 For instance, TMS nanocrystals anchored on two-dimensional (2D) graphene or MXene substrates have been widely developed as electrode materials. 26,27 However, the TMS nanocrystals tend to detach from the 2D substrates. To address these issues, encapsulating TMS nanocrystals into 3D conductive networks has proven to be effective, but limited by ion diffusion. [28][29][30] In light of this, decorating 3D porous nanosparticle-baseds TMS structures with localized carbon additives would satisfy the ion diffusion, electron transmission, and structure stability simultaneously. 31,32 Although effective, it is still a great challenge to control the arrangement of TMS nanoparticles and carbon additives with 3D hierarchical porous structures.
Herein, a nanoemulsion-directed approach to a hierarchical porous hydrangea-like ZnS nanosphere decorated with built-in carbon additives has been developed. In the synthesis, the F127 work as not only a capping agent to control the vertical arrangement of ZnS but also a built-in carbon source to derive the localized carbon additives along with inner mesopores. It should be noted that the spherical arrangement of ZnS with vertical pores radiating out from the center to the surface provides not only 3D self-supported vertical bones to withstand the volume change but also built-in electrolyte-buffering reservoirs to facilitate the ion diffusion into the inner pores of ZnS@C, which distinguished it from other hierarchical porous structures. Besides, the in situ-derived localized carbon not only serves as built-in conductive channels to improve the reaction kinetics of the ZnS nanoparticles but also works as a cross-linking binder to keep their integrity. When evaluated as anode material for SIBs, such a hydrangea-like ZnS@C nanocomposite delivers a high capacity of 620 mAh g −1 at a current of 100 mA g −1 . Even operated at 2 A g −1 for 2000 cycles, it also achieves 286 mAh g −1 .

| Material synthesis
In a typical synthesis, 6 g sodium thiocyanate was first dissolved in a mixed solution containing 30 mL ethylene glycol and 25 mL deionized water. Then, 1 g poly(ethylene glycol)-block-poly(propylene glycol)-blockpoly(ethylene glycol) (F127, PEG-PPG-PEG, Mn~2900) and 5 mL concentrated hydrochloric acid (36%) were added into above mixed system, separately, under vigorous stirring. After F127 powder was dispersed uniformly, 0.268 g Zn(NO 3 ) 2 ·6H 2 O and 3 mL mesitylene were added continuously. After stirring for another 3 min, the resultant mixture was rapidly transferred to a Teflon-lined autoclave and hydrothermally treated at 200°C for 6 h. Then, ZnS@F127/mesitylene nanospheres were acquired by washing with ethanol several times and dried in an oven at 80°C. After thermal treatment with argon (99.9%) at 700°C for 12 h, hierarchical porous hydrangea-like ZnS@C nanospheres were obtained by the in situ carbonization of F127 and mesitylene.

| Material characterization
Scanning electron microscopy (SEM) images were captured by using Verios G4 SEM (FEI Company). Transmission electron microscope (TEM) images were acquired on a Talos F200X microscope (FEI). X-ray diffraction (XRD) patterns were performed on XRD-7000 (Shimadzu). X-ray photoelectron spectroscopy (XPS) was conducted on a Kratos Axis Ultra instrument (DLD). The thermogravimetric curves were collected on a Mettler Toledo TGA/DSC STAR apparatus. Elemental analysis was performed on a Vario EL CHNOS Elemental Analyzer (elementar Analysensysteme GmbH). The infrared spectroscopy curves were carried out on a Bruker Tensor 27 FTIR (Fourier transform infrared) spectrophotometer (Bruker Corporation). The surface area and pore size distribution were calculated by analyzing the N 2 adsorption/desorption isotherms that were recorded on a TriStar II physisorption analyzer (Micromeritics).

| Electrochemical measurement
The electrochemical properties were analyzed by assembling 2025-type coin cells. For the preparation of working electrodes, the as-prepared ZnS@C nanospheres (75 wt%) were mixed with acetylene black (15 wt%) and polyvinylidene fluoride (10 wt%) in N-methyl pyrrolidone under vigorous stirring for 10 h. The acquired slurry was then coated on a piece of copper foil and dried in a vacuum oven overnight. After that, the working electrode was punched into some disks with a diameter of 14 µm with active materials around 1.4 mg cm −2 . When assembling the coin cells in an Ar-filled glovebox the sodium metal piece was worked as the counter electrode, a polypropylene membrane (Celgard 2500) was selected as the separator, and the CF 3 NaO 3 S in diglyme (1.0 M) was utilized as electrolyte. The electrochemical performance was performed on a LAND 2001A Battery Tester between 0.1 and 3.0 V at various current densities. The galvanostatic intermittent titration technique (GITT) test was carried out during the third cycle of charge and discharge of the electrode material. Specifically, a constant current of 100 mA g −1 was applied for 20 min, and then the battery was left in the open circuit state for 2 h. The cyclic voltammetry curves and electrochemical impedance spectroscopy were collected on a CHI750D electrochemical workstation.

| RESULTS
To construct a hierarchical porous nanosphere with penetrating pores, a nanoemulsion assembly approach is developed to direct the vertical arrangement of ZnS. In the synthesis, F127 and mesitylene are selected as a template and mediator, respectively. 16 As schematically depicted in Figure 1, under vigorous stirring, F127 was firstly dispersed uniformly in the acidic ethylene glycol/water system and formed a large number of free micelles ( Figure 1A). After the addition of zinc nitrate, due to the strong coordination force between Zn 2+ ions and F127, Zn 2+ ions will widely exist around F127 micelles and gradually form F127/Zn 2+ micelle aggregates ( Figure 1B). However, the volume of free F127/Zn 2+ micelles or micellar aggregates is relatively small, making it difficult to achieve the purpose of preparing macropores. Therefore, we choose the commonly used pore expander mesitylene to increase the volume of micelles ( Figure 1C). As shown in Figure 1D, under the action of mesitylene, micelles or micellar aggregates are gradually transformed into larger nanoemulsions of F127/mesitylene/Zn 2+ with spherical arrangements radiating outward from the center. During the hydrothermal treatment process, along with the shearing stress due to the in situ generation of ZnS nanocrystals, the nanoemulsion will experience a combined deform and growth process along the preformed radical directions. 16 Then, a hydrangea-like ZnS@F127/ mesitylene nanosphere with penetrating macropores radiating outwards from the center gradually takes shape. After calcination under an argon atmosphere, the carbonaceous F127 and mesitylene are in situ carbonized into built-in carbon additives binding resulting in the formation of hierarchical porous hydrangea-like ZnS@C nanospheres.
As revealed by the SEM images ( Figure 2A and Supporting Information: Figure S1), some monodispersed nanospheres of ZnS@F127/mesitylene with a uniform size of 2 µm are successfully generated on a large scale. Remarkably, as a proof of concept, some penetrating macropores with an apparent diameter HE ET AL.  Figure S2). FTIR spectrum with characteristic absorption peaks derived from -CH 2 (2910, 1248, and 730 cm −1 ) and C-O-C (1105, 1182, and 845 cm −1 ) suggest the presence of F127 and mesitylene along with ZnS ( Figure 2C). 33,34 The XPS survey spectra further confirm the coexistence of Zn, S, and C elements. In detail, the high-resolution XPS spectrum of C 1s could be deconvoluted into three chemical environments of carbon at 288.5 and 286 eV for ether carbon and 284.6 eV for alkyl carbon, respectively ( Figure 2D). 35,36 In terms of Zn 2p, the two peaks at 1021.1 and 1044.1 eV are ascribed to Zn 2p 1/2 and Zn 2p 3/2 of Zn 2+ , respectively ( Figure 2E). Besides, the XPS spectrum of S 2p can be fitted into two peaks of S 2p 3/2 at 161 and S 2p 1/2 at 162.1 eV, respectively ( Figure 2F). 37 All of these results indicate that a hydrangea-like nanosphere of ZnS@F127/mesitylene with penetrating macropores radiating out from the center to the surface has been achieved. It should be noted that the concentrations of Zn 2+ ions and mesitylene work together to rationally control the spherical structure with the vertical arrangement. 16 In terms of Zn 2+ ions, excessive Zn 2+ ions lead to the shrinking of the penetrated pores, resulting in the generation of solid nanospheres with increased size but without penetrating pores (Supporting Information: Figure S3). Although some sunken macropores are still present in some spheres, TEM images reveal that they just exist on the surface with limited depth like a golf ball (Supporting Information: Figure S4). This phenomenon can be attributed to the enriched Zn 2+ ions around F127/ mesitylene and the subsequent rapid growth of ZnS nanoparticles. In contrast, when increasing mesitylene's amount, some decreased nanospheres in size with irregular hollow cavities arise in the products (Supporting Information: Figures S5 and S6). This is because the mesitylene working as the mediator could accelerate the adsorption of Zn 2+ ions along lateral sides more than vertical sides. Therefore, F127 works together with mesitylene and plays a key role in directing the vertical arrangement of ZnS nanocrystals.
Owing to the homogeneous distribution of F127/ mesitylene between ZnS nanocrystals, they could further act as the in situ carbon source to derive the built-in functional carbon additives. As a result, after calcination under an argon atmosphere (Supporting Information: Figure S7), the hydrangea-like structure is well maintained without any shrinking ( Figure 3A,B), which benefit from the cushioning effect of the hierarchical porous structure. XRD pattern suggests the wellmaintained ZnS with high crystallinity ( Figure 3C). The two characteristic peaks centered at 1345 and 1580 cm −1 of the Raman spectrum corresponds to the D and G band of carbon, respectively, which verify the successful carbonization of F127 and mesitylene ( Figure 3D), which is in agreement with the FTIR spectrum without any adsorption peaks of F127 and mesitylene (Supporting Information: Figure S8). Besides, the peak at 286 eV for C-O-C disappears after calcination, which further confirms the full carbonization of F127 riching in ether bonds (Supporting Information: Figure S9). To better understand the microstructure of the hydrangea-like ZnS@C nanospheres, TEM is further performed to reveal their internal texture. In line with our original expectations, the nanospheres demonstrate the vertical arrangement of ZnS with fan-like macropores radiating out from the center to the surface ( Figure 4A and Supporting Information: Figure S10). In view of complementary, as shown in Figure 4B, the radicalized arrangement of ZnS skeleton like the inner-added self-supported bones to withstand the integrity of the nanosphere from both lateral and vertical directions. Furthermore, the main body of the nanospheres is assembled from ultrasmall ZnS nanoparticles with a size of around 10 nm ( Figure 4C,D). The selected area electron diffraction pattern demonstrates a set of distinct diffraction rings matching (101), (108), (109), (209), and (218) planes of hexagonal ZnS ( Figure 4E), respectively, which is consistent with the XRD result. The well-aligned lattice fringes with an interplanar spacing of 0.326 and 0.286 nm shown in the high-resolution TEM image could be assigned to the (101) and (103) planes of ZnS as well ( Figure 4F). Besides ZnS, it should be noted that some carbon belts obviously distribute along with the ZnS nanocrystal (within the dashed line in Figure 4F). This result directly proves the oriented function of F127 and mesitylene in controlling the vertical arrangement of ZnS, which could further work as an implanted carbon source to derive the built-in carbon additives. Besides, as shown in the STEM elemental mapping, the distribution of C accords well with Zn and S, which confirms the uniform distribution of  carbon along with ZnS nanoparticles ( Figure 4G). From the energy-dispersive X-ray spectroscopy spectra (Supporting Information: Figure S11), we can further confirm the presence of carbon in the calcined product. By analyzing with a chemical element analyzer, the weight percentage of carbon in ZnS nanospheres is around 1.86% (Supporting Information: Table S1). The mass fraction of Zn element in the material is about 42% by inductively coupled plasmaatomic emission spectroscopy (ICP-AES) test. Due to the stress relief, some pores could be generated during the carbonization process of F127 and mesitylene. As a result, by analyzing the nitrogen isotherms based on the Brunauer-Emmett-Teller (BET) theory (Supporting Information: Figure S12), the ZnS@C nanospheres possess mesopores with a diameter of around 10 nm and a specific surface area of 52 cm 2 g −1 . In addition, the macropore size calculated from the mercury intrusion is characterized by a fan-shaped distribution centered at 208 nm (Supporting Information: Figure S13), which evidently demonstrates the cone-shaped vertical pores radiating out from the center to the surface. By reason of the foregoing, a nanoemulsion-directed method has been successfully achieved to direct the vertical arrangement of ZnS nanospheres with both vertically arranged cone-shaped macropores and inner mesoporous.
In light of their hierarchical porous structure with built-in carbon additives, these hydrangea-like ZnS@C nanospheres are studied as anode materials for SIBs. First, the cyclic voltammogram (CV) curves ( Figure 5A) are performed at the potential window of 0.1-3.0 V (vs. Na/Na + ) to reveal their conversion storage mechanism. Consequently, the reduction peaks appearing at 0.35 and 0.11 V in the first cathodic scanning process could be assigned to the irreversible formation of the solid-electrolyte interface (SEI) layer and the reduction of ZnS, respectively. 38,39 Whereas the oxidation peak at 0.88 V in the subsequent anodic scan can be attributed to the reverse formation of ZnS. As the cycle goes on, the cathodic peak at 0.35 V broadens into 0.5 V due to the conversion of ZnS to Zn. Starting from the third cycle, the CV curves are well overlapped, suggesting good reversibility with the conversion mechanism. Then, galvanostatic discharge/charge measurement at a constant current density of 100 mA g −1 is carried out to evaluate their Na + ion storage capability ( Figure 5B). As a result, these ZnS@C nanospheres deliver an initial discharge capacity of 885 mAh g −1 with a coulombic efficiency of 78.5%. The irreversible capacity arises from the formation of SEI film. In the following four cycles, their discharge capacity stays around 620 mAh g −1 with a high coulomb efficiency of 95%. It is anticipated that the hierarchical porous structure along with the carbon additives could work together to facilitate the reaction kinetics. As shown in Figure 5C, the ZnS@C nanospheres deliver reversible capacities of 581.6, 515.5, 403.8, 315.5, 242.7, and 181.8 mAh g −1 at the current densities of 100, 200, 400, 600, 1000, and 2000 mA g −1 , respectively. As the cycling rate is set back to 100 mA g −1 , a discharge capacity of 426.1 mAh g −1 is immediately recovered, exhibiting good rate capability.
To further evaluate its practical application prospect, the long-term cycling performance of ZnS@C nanospheres is conducted at the current densities of 1.0 and 2.0 A g −1 , respectively. Impressively, after cycling 1000 cycles at 1.0 A g −1 , the ZnS@C nanospheres deliver a high capacity of 320 mAh g −1 with an average coulombic efficiency of 100% ( Figure 5D). When the current density is further increased to 2 A g −1 , a short period of capacity decline appears due to the inadequate activation of the internal active material caused by the faster ion intercalation-deintercalation behavior. After that, a constant reversible discharge capacity of 286 mAh g −1 is achieved even after 2000 cycles ( Figure 5D). By carefully examing the nanostructure after cycling, the hydrangealike structure with penetrating pores radiating out from the center to the surface is well maintained (Supporting Information: Figures S14 and S15). The XRD data of the material after cycling also show that the material has good stability (Supporting Information: Figure S16). The high-rate capability of the hydrangea-like ZnS@C nanospheres can be ascribed to the following merits. First, the ultra-small ZnS nanoparticles could offer much more active sites with low energy barriers for Na + ion storage, which greatly increases the capacity utilization of ZnS bulks. Second, the vertically aligned macropores radiating out from the center to the surface could work as a built-in electrolyte-buffer reservoirs to facilitate the ion diffusion into the inner mesoporous of ZnS@C nanospheres, which greatly improves the rate performance by shortening the ion diffusion distance. Third, the mesoporous ZnS skeletons decorated with carbon binders could serve as the self-supported vertical bones to withstand the volume change from both lateral and vertical sides, which contribute significantly to the longcycle performance. Last but not least, the localized carbon additives distributed among the ZnS nanoparticles make the radical bones effective 3D conductive channels to facilitate electron transmission, which further increases the reaction kinetics effectively. Benefiting from the above-mentioned synergistic effects, the long-term cycling performance of the as-prepared hydrangea-like ZnS@C nanospheres at high rates is superior to many of the published performances of ZnSbased materials (Supporting Information: Table S2).
To collect more information about the reaction mechanism, a series of ex situ XRD patterns performed at different discharge and charge states are collected to examine the crystal evolution of ZnS@C nanospheres during the first discharge-charge process. As shown in Figure 6, the peak evolutions at 27°, 28.5°, 47.4°, and 56.1°can be indexed to the crystal change of ZnS. During the initial discharge stage, the intensity of these peaks gradually decreases accompanied by the increase of Na 2 S via NaS intermediates. These results indicate the conversion mechanism of ZnS via the reversible process of ZnS + Na + + e − → NaS + Zn and NaS + Na + + e − → Na 2 S. During the charging process, the Na + ions are gradually released from Na 2 S via NaS intermediates, while ZnS nanocrystals are gradually recovered. In light of this, it is believed that the hydrangea-like ZnS@C nanospheres could work as efficient anode materials with a conversion mechanism for Na ions storage.
To further understand the electrochemical kinetics of the hydrangea-like ZnS@C nanospheres towards Na + ion storage, CV curves are collected at different scanning rates from 0.2 to 1.0 mV s −1 . As shown in Figure 7A, as the scan rate rise, both currents of cathodic and anodic peaks grow steadily with less obvious peak shift, which suggests the fast reaction kinetics of ZnS@C nanospheres with low polarization during the Na + storage process. It is believed that the relationship between current (i) and scan rate (v) keeps in line with the following equations 40,41 : It is anticipated that the b value can be utilized to distinguish between the Faradaic intercalation process (b ≈ 0.5) and the capacitive-controlled process (b ≈ 1.0) involved in the Na + ions storage mechanism. As a result, the b values for both cathodic and anodic are all around 0.7 ( Figure 7B). The ZnS@C nanospheres experience a combined contribution from both Faradaic intercalation and capacitive kinetics. Besides, the capacitive contribution gradually increases from 58.6% to 75.3% as the rising of scan rate from 0.2 to 1.0 mV s −1 ( Figure 7C and Supporting Information: Figure S17). Such a high capacitive contribution can be ascribed to the hierarchical porous structure of ZnS@C nanospheres with both penetrating macropores and cross-linked niner mesoporous, which is also responsible for the excellent rate performance. To illustrate the solid-state diffusion kinetics of Na + in this hydrangea-like hierarchical porous structure, the GITT is applied to calculate the chemical diffusion coefficient of Na + (D Na+ ). As shown in Figure 7D, the shape of the GITT curves under pulse operation in the third cycle is in keeping with the discharge-charge profiles under continuous conditions. This result indicates that the work state of ZnS@C nanospheres is close to the equilibrium state even in the continuous discharge-charge process, which suggests their enhanced electronic and ionic conductivity. [42][43][44] Since the cell voltage is linearly proportional to the onehalf of the pulse time (Supporting Information: Figure S18), the D Na+ values are obtained according to Fick's second law. [45][46][47] As shown in Figure 7E, the calculated D Na+ for ZnS@C nanospheres ranges from 10 −11 to 10 −8 cm 2 s −1 , which reflects the benefits of the hierarchical porous nanostructure comprising of penetrating macropores from the center to the surface and inner mesoporous distributed among the ZnS nanocrystals. Besides, the charge-transfer resistances of the ZnS@C nanospheres decreased from 29.44 to 9.98 Ω after cycling F I G U R E 6 Ex situ X-ray diffraction patterns of ZnS@C nanospheres at different discharge and charge states during the first dischargecharge process. 300 cycles at 500 mA g −1 , indicating the rapid interface dynamics upon cyclings. Consequently, by coupling the merits of ultrasmall ZnS nanocrystals, inner-added carbon additives, and hierarchical porous structure, a decent anode material of hierarchical porous ZnS@C nanospheres that delivers high-rate capacity and long cycle life for SIBs has been successfully achieved.

| CONCLUSION
In summary, we present a nanoemulsion assembly approach toward hierarchical porous ZnS@C nanospheres with penetrating pores radiating out from the center to the surface. When evaluated as an anode material for SIBs, such a newly developed 3D hydrangea-like ZnS@C nanocomposite combines several strengths for Na + ion storage. First, the penetrated pores could work as built-in electrolyte-buffer reservoirs to facilitate the ion diffusion into the inner mesopores of ZnS@C nanospheres, which greatly shortens the ion diffusion distance. Second, the radical porous structure decorated with carbon binders could serve as the self-supported vertical bones to withstand the volume change from both lateral and vertical sides, keeping their integrality consistent. Third, the localized carbon distributed among the ZnS nanocrystal makes the radical bones as effective 3D conductive channels to facilitate electron transmission. As a result, such hydrangea-like ZnS@C nanospheres exhibit superior electrochemical sodium storage properties (320 mAh g −1 at a current of 1 A g −1 after 1000 cycles and 286 mAh g −1 at 2 A g −1 after 2000 cycles). It is anticipated that the nanoemulsion-directed approach reported here paves a new way to design hierarchical porous nanocomposites for post-lithium-ion batteries.