Three-Dimensionally Ordered Macro/Mesoporous Nb2O5/Nb4N5 Heterostructure as Sulfur Host for High-Performance Lithium/Sulfur Batteries

The severe shuttle effect of soluble polysulfides hinders the development of lithium–sulfur batteries. Herein, we develop a three-dimensionally ordered macro/mesoporous (3DOM) Nb2O5/Nb4N5 heterostructure, which combines the strong adsorption of Nb2O5 and remarkable catalysis effect of Nb4N5 by the promotion “adsorption-transformation” mechanism in sulfur reaction. Furthermore, the high electrocatalytic activity of Nb4N5 facilitates ion/mass transfer during the charge/discharge process. As a result, cells with the S-Nb2O5/Nb4N5 electrode delivered outstanding cycling stability and higher discharge capacity than its counterparts. Our work demonstrates a new routine for the multifunctional sulfur host design, which offers great potential for commercial high-performance lithium–sulfur batteries.


Introduction
Electronic devices play a vital role in modern society, setting high standards for corresponding energy storage systems [1,2]. Lithium-sulfur batteries (LSBs) are demonstrated as one of the most promising candidates owing to their high theoretical energy density, low cost, and environmental friendliness [3][4][5]. However, the solid-electrolyte interphase (SEI) has been found to have poor mechanical strength and Li-ion conductivity. The formation of unstable SEI causes safety issues and faster decay of capacity in the anode side for LSB [6]. Artificial SEI fabricated by fluorinated electrolyte and ultrathin bilayer SEI are thus applied to protect the electrodes and suppress Li dendrite growth [7,8]. Besides, the development of LSBs is hindered by low conductivity of sulfur and its discharge product, repeated volume change, and severe lithium polysulfides (LiPS) shuttle effect [9][10][11][12].
To enhance the performance of LSBs, several kinds of sulfur hosts have developed by researchers, including carbon materials, conductive polymer, and metallic chalcogenides, among others [13][14][15][16][17]. A sulfur host can fasten charge transfer in LSB, capture LiPS, and catalyze each step of conversion of this chemical species. Qiao et al. combined iron phosphide (FeP) with reduced graphene oxide (rGO) to construct a sulfiphilic composite [18]. The catalytic properties of FeP and the electron transport properties of rGO are integrated by the synergistic effect, which results in high coulombic efficiency and capacity of the cell loaded with this kind of sulfiphilic host. Recently, polar metal oxides were found to deliver great potential to serve as a sulfur host relying on polar-polar interactions with the LiPS [19][20][21]. As an oxygen-rich material, anions of metal oxides work as active sites to absorb the LiPS. Among the family of oxides, Nb 2 O 5 shows high LiPS anchor ability due to the strong metal-sulfur bond [22,23]. However, as an insulator with a wide band gap energy, the Nb 2 O 5 sulfur host is still limited by poor electronic conductivity. While, transition metal nitride is reported to possess high conductivity and catalytic activity, and have been widely used as a sulfur host for LSBs [24][25][26][27]. When different catalysts are applied in LSB, them can catalyze various processes in the conversion of LiPS. Wang et al. found that FeP is able to catalyze the liquid-liquid-solid process, while Fe 3 O 4 can promote the solid-liquid conversion. When the batteries were assembled with these two catalysts, the cycle stability and capacity retention of the battery was improved simultaneously [28]. Hence, it is a feasible way to combine the Nb 2 O 5 and transition metal nitride to achieve a high-performance sulfur host.
Herein, we develop a three-dimensionally ordered macro/mesoporous (3DOM) Nb 2 O 5 / Nb 4 N 5 heterostructure, which combines the strong adsorption of Nb 2 O 5 and remarkable catalysis effect of Nb 4 N 5 , promoting the "adsorption-transformation" mechanism in the lithium-sulfur battery. Furthermore, the high electrocatalytic activity of Nb 4 N 5 can provide a fast ion transfer routine during the cycling process and the ordered porous structure not only provides sufficient space for sulfur loading, but also improves the electrolyte infiltration. Therefore, the S-Nb 2 O 5 /Nb 4 N 5 electrode delivers satisfying cycling stability and remarkable discharge capacity.

Materials Preparation
The polymethyl methacrylate (PMMA) template was prepared according to our reported methods [29]. In the typical procedure of the synthesis of 3DOM Nb 2 O 5 /Nb 4 N 5 , 20 mL ethanol was mixed with 1.35 g of niobium pentachloride (NbCl 5 , Aladdin, Shanghai, China) under magnetic stirring. When a clear solution was formed, the prepared PMMA template was immersed in the precursor solution for 12 h. Subsequently, the precursor solution was removed from the PMMA template through vacuum filtration. The obtained sample was put into a porcelain boat and dried in air at 60 • C. Subsequent calcination at 600 • C in air for 3 h was employed to remove the PMMA template. The obtained 3DOM Nb 2 O 5 was heated under NH 3 to prepare 3DOM Nb 2 O 5 /Nb 4 N 5 .

Cell Assembling and Testing
All reagents for assembling and testing are purchased from Aladdin (Shanghai, China) without further purification. At a mass ratio of 75:25 (wt.%), Nb 2 O 5 /Nb 4 N 5 and sulfur powder were ground together and melting-diffusion routine was conducted to obtain S-Nb 2 O 5 /Nb 4 N 5 . N-hydroxy-2-pyrrolidone (NMP) was used as a solvent, and S-Nb 2 O 5 /Nb 4 N 5 and conductive carbon black with polyvinylidene fluoride powder (PVDF) were mixed to produce a black slurry, in a mass ratio of 8:1:1. Al foil, serving as current collector, was coated with the as-prepared slurry and dried at 60 • C overnight. CR-2032 coin-type cells were applied to study the prepared electrode. To assemble the cells, cathode was made with a diameter of 12 mm. In this kind of cell, Li foil is an anode while Celgard 2400 works as a separator. 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) (v/v = 1/1) were mixed with 1 M LiTFSI and 1% LiNO 3 additive to serve as an electrolyte of the cell. Cyclic voltametric test and electrochemical impedance spectroscopy were carried out on a CHI660E (CH Instruments, Inc., Austin, TX, USA) electrochemical workstation. Charge-discharge surveys were conducted on a Neware battery tester (Shenzhen, China) from 1.7 V to 2.8 V.
LiPS adsorption test: The polysulfides were produced by adding sulfur and lithium sulfides in DME at a specific molar ratio. All tested samples was mixed with diluted Li 2 S 6 solution inside the glove box. The photographs of absorption result were collected after stirring for 2 h and aging for one-sixth of a day.
Symmetric cells: 0.5 mg cm −2 of active materials were dropped onto the circular disks (12 mm in diameter) of carbon cloths. The amount of 0.2 M Li 2 S 6 solution used in symmetric cells was 30 µL. CV measurements were obtained on a CHI660E (CH Instruments, Inc., Austin, TX, USA) electrochemistry workstation from −0.8 to 0.8 V in 1 mV s −1 .
Li 2 S nucleation test: 0.25 M Li 2 S 8 electrolyte was prepared for the test. Nb 2 O 5 /Nb 4 N 5 heterostructure materials/carbon cloth and the lithium metal were used as electrodes. During the test, all batteries were discharged at 2.06 V with a steady current of 0.112 mA, and then maintained at 2.05 V until the current was decreased to 10 −5 A.
Linear sweep voltammetry (LSV) test: To investigate the oxidization behavior of Li 2 S, LSV measurements were performed in methanol with 0.1 M Li 2 S. Typically, to construct a three-electrode system, an Ag/AgCl electrode and platinum wire are used as the reference electrode and counter electrode, respectively. Moreover, the glass carbon electrode covered with prepared materials was used as the working electrode. The prepared materials were dispersed in NMP and added onto a glass carbon electrode to fabricate a working electrode. The tests were conducted by scanning from −0.4 to −0.2 V in 5 mV s −1 .

Results and Discussion
As observed in Figure 1, NbCl 5 first penetrated the closed-packing PMMA template by capillarity forces. Subsequently, the PMMA was removed by heating at 600 • C, thus by the formation of 3DOM Nb 2 O 5 . The construction of a heterostructure relied on the Nb 2 O 5 nitridation treatment through NH 3 erosion at a high temperature. When the conversion of the LiPSs species occurs, satisfactory pores and voids were provided by the unique 3DOM structure, for storing and immobilizing sulfur. Working as a polar material, the Nb 2 O 5 part in the heterojunction can strongly and chemically adsorb polysulfide. Soon after adsorption, the niobium nitride (Nb 4 N 5 ) in the heterojunction enhances the catalysis of polysulfide and solid Li 2 S nucleation is seen.  Figure 2a shows that the composites have three-dimensionally ordered porous structure and the pores are uniformly distributed and interconnected. The pore diameter is around 150-200 nm ( Figure 2b) and sulfur can be absorbed intensely by these nanopores during galvanostatic charge-discharge cycling. Figure S1 shows the energy dispersive spectroscopy (EDS) result. Through  Figure 2a shows that the composites have three-dimensionally ordered porous structure and the pores are uniformly distributed and interconnected. The pore diameter is around 150-200 nm ( Figure 2b) and sulfur can be absorbed intensely by these nanopores during galvanostatic charge-discharge cycling. Figure S1 shows the energy dispersive spectroscopy (EDS) result. Through the analysis of this mapping, the composites have 24% Nb 2 O 5, while the other 76% is Nb 4 N 5 . The TEM result further confirms the 3DOM structure of Nb 2 O 5 /Nb 4 N 5 in Figure 2c, and implies that the unique structure is preserved during the formation of the Nb 2 O 5 /Nb 4 N 5 heterostructure. More importantly, the pore diameter is 160 nm, which is consistent with the SEM image.  Figure 3a; all the peaks are consistent with Nb4N5 (PDF#51-1327) and Nb2O5 (PDF#30-0873), indicating the high purity of the synthesized products. The 3DOM structures are further probed through the N2 adsorption/desorption isotherms ( Figure 3b); a similar specific surface area is obtained by Nb4N5 (34.6 m 2 g −1 ), Nb2O5/Nb4N5 (38.9 m 2 g −1 ), and Nb2O5 (40.9 m 2 g −1 ), indicating that the pore structures are well preserved during the phase transformation process. The pore distributions in Figure 3c indicate the existence of abundant micropores and mesopores, which can adsorb LiPS by relying the physical effect. Furthermore, the pore volume in Table S3 indicate that Nb2O5/Nb4N5 has a large number of bigger pores, which benefit the storing of sulfur. The XPS test is conducted to explore the chemical bonding indicating that the pore structures are well preserved during the phase transformation process. The pore distributions in Figure 3c indicate the existence of abundant micropores and mesopores, which can adsorb LiPS by relying the physical effect. Furthermore, the pore volume in Table S3 indicate that Nb 2 O 5 /Nb 4 N 5 has a large number of bigger pores, which benefit the storing of sulfur. The XPS test is conducted to explore the chemical bonding environment of Nb 2 O 5 /Nb 4 N 5 . Correction for specimen charging is applied to XPS analysis according to the C 1 s peak at 284 eV. Typical Nb-O (209.9 eV, 207.3 eV) and Nb-N (206.8 eV) bonds are obtained (Figure 3d), indicating the co-existence state of Nb 2 O 5 and Nb 4 N 5 . The high-resolution N 1s spectrum exhibits one major peak at 396.4 eV, which can be owing to the existence of Nb-N bonds of Nb 4 N 5 (Figure 3e) [30]. Furthermore, one other major peak appears at 530.6 eV, which can be ascribed to Nb-O bonds of Nb 2 O 5 , and a sub peak of O-containing surface group emerged at 531.5 eV (Figure 3f) [22]. These results further confirm the successful construction of the Nb 2 O 5 /Nb 4 N 5 heterostructure, which is expected to possess high adsorption and catalysis ability for LiPS. The LiPS adsorption effect of Nb2O5/Nb4N5 is evaluated by the LiPS adsorption test using Li2S6 as a representative LiPS ( Figure 4a). As is shown in this photograph, the glass bottles starting from left to right contained blank Li2S6 solution, 3DOM Nb2O5 with Li2S6 solution, 3DOM Nb2O5/Nb4N5 with Li2S6 solution, and 3DOM Nb4N5 with Li2S6 solution, respectively. The orange colour in the solution changed to a lighter brown after 3DOM Nb4N5 was added. Moreover, after 3DOM Nb2O5 is added into the solution, the color of the solution fades intensely and become much more transparent than that of the one with 3DOM Nb4N5, verifying that Li2S6 adsorption ability of Nb2O5 is much stronger than that of Nb4N5. At the same time, the solution with Nb2O5/Nb4N5 became completely colorless, suggesting the synergistic effect of three-dimensionally ordered porous Nb2O5 and the unique catalytic nature of Nb4N5 towards effective trapping of lithium polysulfides. The UV-vis curves comparison displays the vanishing of typical peaks related to S6 2− and S4 2− , demonstrating the strong adsorption of Nb2O5/Nb4N5. The highest current response delivered by the Nb2O5/Nb4N5 electrode can be observed in the LSV test (Figure 4b), indicating the enhanced Li2S oxidation kinetics achieved by Nb2O5/Nb4N5. This result also implies reduction of the energy barrier of conversion of polysulfides by heterojunction, ensuring that the 3DOM Nb2O5/Nb4N5 electrodes promoted catalytic process of sulfur [31]. The TGA graph was performed as shown in Figure S2. The S content can reach 73% owing to the abundant hierarchical pore structure. To assess the enhanced electrochemical kinetics in depth, cyclic voltammetry (CV) characterization of the symmetric cells containing the 0.2 M Li2S6 electrolyte are performed with the scan rate of 1 mV s −1 (Figure 4c). The CV profile of 3DOM Nb2O5/Nb4N5 exhibits excellent reversibility, with two pairs of redox peaks (−0.03, 0.03 and −0.22, 0.22 V) probed. However, the CV of 3DOM Nb2O5 and Nb4N5 only obtains one pair of broadened redox reaction peaks, which is at −0.19 V and 0.19 V, separately. Moreover, the peak intensity of Nb2O5/Nb4N5 is higher than that of Nb2O5 and Nb4N5, indicating the limited transformation of polysulfides on the bare surface of The LiPS adsorption effect of Nb 2 O 5 /Nb 4 N 5 is evaluated by the LiPS adsorption test using Li 2 S 6 as a representative LiPS (Figure 4a). As is shown in this photograph, the glass bottles starting from left to right contained blank Li 2 S 6 solution, 3DOM Nb 2 O 5 with Li 2 S 6 solution, 3DOM Nb 2 O 5 /Nb 4 N 5 with Li 2 S 6 solution, and 3DOM Nb 4 N 5 with Li 2 S 6 solution, respectively. The orange colour in the solution changed to a lighter brown after 3DOM Nb 4 N 5 was added. Moreover, after 3DOM Nb 2 O 5 is added into the solution, the color of the solution fades intensely and become much more transparent than that of the one with 3DOM Nb 4 N 5 , verifying that Li 2 S 6 adsorption ability of Nb 2 O 5 is much stronger than that of Nb 4 N 5 . At the same time, the solution with Nb 2 O 5 /Nb 4 N 5 became completely colorless, suggesting the synergistic effect of three-dimensionally ordered porous Nb 2 O 5 and the unique catalytic nature of Nb 4 N 5 towards effective trapping of lithium polysulfides. The UV-vis curves comparison displays the vanishing of typical peaks related to S 6 2− and S 4 2− , demonstrating the strong adsorption of Nb 2 O 5 /Nb 4 N 5 . The highest current response delivered by the Nb 2 O 5 /Nb 4 N 5 electrode can be observed in the LSV test (Figure 4b), indicating the enhanced Li 2 S oxidation kinetics achieved by Nb 2 O 5 /Nb 4 N 5 . This result also implies reduction of the energy barrier of conversion of polysulfides by heterojunction, ensuring that the 3DOM Nb 2 O 5 /Nb 4 N 5 electrodes promoted catalytic process of sulfur [31]. The TGA graph was performed as shown in Figure S2. The S content can reach 73% owing to the abundant hierarchical pore structure. To assess the enhanced electrochemical kinetics in depth, cyclic voltammetry (CV) characterization of the symmetric cells containing the 0.2 M Li 2 S 6 electrolyte are performed with the scan rate of 1 mV s −1 (Figure 4c). The CV profile of 3DOM Nb 2 O 5 /Nb 4 N 5 exhibits excellent reversibility, with two pairs of redox peaks (−0.03, 0.03 and −0.22, 0.22 V) probed. However, the CV of 3DOM Nb 2 O 5 and Nb 4 N 5 only obtains one pair of broadened redox reaction peaks, which is at −0.19 V and 0.19 V, separately. Moreover, the peak intensity of Nb 2 O 5 /Nb 4 N 5 is higher than that of Nb 2 O 5 and Nb 4 N 5 , indicating the limited transformation of polysulfides on the bare surface of Nb 2 O 5 /Nb 4 N 5 heterojunction. The initial three CV cycles (Figure 4d) of the Nb 2 O 5 /Nb 4 N 5 heterojunction are perfectly overlapped, suggesting excellent cycling stability. As is widely accepted by the scientific community, the transformation from Li 2 S 4 to Li 2 S contributes almost 75% of discharge capacity. As a result, the Li 2 S deposition test is conducted, as shown in Figure 4e. It can be observed from Figure 4e that nucleation peak response of the heterostructure is earlier than that of Nb 2 O 5 and Nb 4 N 5 , and nucleation capacity of 3DOM Nb 2 O 5 /Nb 4 N 5 (283 mAh g −1 ) is highest among the three samples. This may lead to a lower overpotential of the nucleation, electrocatalytic conversion of Li 2 S, and adsorbent to polysulfide species [32]. The electrochemical performance is tested by employing the S-Nb2O5/Nb4N5 electrode. The CV results are shown in Figure 4f-two distinct reduction peaks and one main oxide peak can be seen. The reduction peaks located at ~2.35 V and ~2.05 V represent the transformation from sulfur to Li2S4 and further into Li2S. The oxide peak is produced by the regeneration of sulfur. The nearly overlapped curves indicate the excellent reversibility of the electrochemical reactions. Figure S3 show the Nyquist plots of the battery loaded with different samples in the frequency range 0.01-100 KHz. The S-Nb2O5/Nb4N5 cathode shows the smallest charge-transfer resistance (Rct), denoting its fast kinetic process. The internal resistance (Rs) of all samples is similar and the Rct of S-Nb2O5/Nb4N5 cathode is 46.79 Ω (Table S2).Furthermore, a long-term cycling test at 1 C was conducted (Figure 4g). The first 2 cycles at 0.2 C are applied for activation of the electrodes. A high discharge capacity of 1354 mAh g −1 at 1 C is obtained and a remarkable reversible capacity of 913 mAh g −1 can still be maintained after 400 cycles with a low capacity attenuation rate (0.08% per cycle), which is obviously improved compared with the S-Nb2O5 and S-Nb4N5 electrodes. Additionally, the voltage profiles at 1 C are provided in Figure S4; it can be seen that the S-Nb2O5/Nb4N5 electrode displays stable voltage plateau and negligible polarization behavior under prolonged cycling. Moreover, cycling test at 1 C with different mass loading is shown in Figure S5. The as-developed S-Nb2O5/Nb4N5 electrodes are capable of withstanding at 1 C at sulfur loading of 2 and 6 mg cm −2 ( Figure S5), attributing the favorable mass/charge transfer and the catalyzed sulfur redox reactions in the Nb2O5/Nb4N5 The electrochemical performance is tested by employing the S-Nb 2 O 5 /Nb 4 N 5 electrode. The CV results are shown in Figure 4f-two distinct reduction peaks and one main oxide peak can be seen. The reduction peaks located at~2.35 V and~2.05 V represent the transformation from sulfur to Li 2 S 4 and further into Li 2 S. The oxide peak is produced by the regeneration of sulfur. The nearly overlapped curves indicate the excellent reversibility of the electrochemical reactions. Figure S3 show the Nyquist plots of the battery loaded with different samples in the frequency range 0.01-100 KHz. The S-Nb 2 O 5 /Nb 4 N 5 cathode shows the smallest charge-transfer resistance (R ct ), denoting its fast kinetic process. The internal resistance (R s ) of all samples is similar and the R ct of S-Nb 2 O 5 /Nb 4 N 5 cathode is 46.79 Ω (Table S2).Furthermore, a long-term cycling test at 1 C was conducted (Figure 4g). The first 2 cycles at 0.2 C are applied for activation of the electrodes. A high discharge capacity of 1354 mAh g −1 at 1 C is obtained and a remarkable reversible capacity of 913 mAh g −1 can still be maintained after 400 cycles with a low capacity attenuation rate (0.08% per cycle), which is obviously improved compared with the S-Nb 2 O 5 and S-Nb 4 N 5 electrodes. Additionally, the voltage profiles at 1 C are provided in Figure S4; it can be seen that the S-Nb 2 O 5 /Nb 4 N 5 electrode displays stable voltage plateau and negligible polarization behavior under prolonged cycling. Moreover, cycling test at 1 C with different mass loading is shown in Figure S5. The as-developed S-Nb 2 O 5 /Nb 4 N 5 electrodes are capable of withstanding at 1 C at sulfur loading of 2 and 6 mg cm −2 ( Figure S5), attributing the favorable mass/charge transfer and the catalyzed sulfur redox reactions in the Nb 2 O 5 /Nb 4 N 5 matrix. On comparison of our work with other current works, it is seen that the Nb 2 O 5 /Nb 4 N 5 electrode exhibits excellent electrochemical performance among recently published heterojunction materials for LSB (Table S4).

Conclusions
A 3DOM Nb 2 O 5 /Nb 4 N 5 heterostructure was constructed through in-situ nitridation to serve as a multi-functional sulfur host. The porous structure with interconnected channels can accommodate sulfur as well as facilitate electrolyte infiltration. Strong LiPS immobilization of Nb 2 O 5 and the remarkable catalysis effect of Nb 4 N 5 are combined to realize the accelerated LiPS "adsorption-transformation" process. As a result, the LSBs with S-Nb 2 O 5 /Nb 4 N 5 delivered enhanced kinetics and improved cycling stability and discharge capacity, indicating great capability of Nb 2 O 5 /Nb 4 N 5 for high-performance LSBs.