Ultrathin NiO/Ni3S2 Heterostructure as Electrocatalyst for Accelerated Polysulfide Conversion in Lithium–Sulfur Batteries

The practical application of Lithium–Sulfur batteries largely depends on highly efficient utilization and conversion of sulfur under the realistic condition of high‐sulfur content and low electrolyte/sulfur ratio. Rational design of heterostructure electrocatalysts with abundant active sites and strong interfacial electronic interactions is a promising but still challenging strategy for preventing shuttling of polysulfides in lithium–sulfur batteries. Herein, ultrathin nonlayered NiO/Ni3S2 heterostructure nanosheets are developed through topochemical transformation of layered Ni(OH)2 templates to improve the utilization of sulfur and facilitate stable cycling of batteries. As a multifunction catalyst, NiO/Ni3S2 not only enhances the adsorption of polysulfides and shorten the transport path of Li ions and electrons but also promotes the Li2S formation and transformation, which are verified by both in‐situ Raman spectroscopy and electrochemical investigations. Thus, the cell with NiO/Ni3S2 as electrocatalyst delivers an area capacity of 4.8 mAh cm−2 under the high sulfur loading (6 mg cm−2) and low electrolyte/sulfur ratio (4.3 μL mg−1). The strategy can be extended to 2D Ni foil, demonstrating its prospects in the construction of electrodes with high gravimetric/volumetric energy densities. The designed electrocatalyst of ultrathin nonlayered heterostructure will shed light on achieving high energy density lithium–sulfur batteries.


Introduction
The demand of energy storage cells with high energy density is significantly increasing due to the market expansion of electric vehicles, portable electronics, and smart gird storage in recent years. [1]Lithium-sulfur (Li-S) batteries are supposed to be one of the most promising candidates for next-generation energy storage systems owing to their high theoretical energy density of 2567 Wh kg −1 achieved by matching the sulfur cathodes with lithium metal anodes. [2,3]Unfortunately, the insulating property of sulfur species and the shuttling effect of lithium polysulfides (LiPSs) lead to the poor cycling stability under the high sulfur loading, which hinders the widespread practical implementation of Li-S batteries. [4,5]The fundamental reason lies in the sluggish LiPSs conversion kinetics, which leads to continued accumulation of LiPSs, exacerbating the shuttling effect.
Rational utilization of electrocatalysts is crucial in accelerating the LiPSs conversion and mitigating their shuttling effect.To achieve above effects, the catalysts should possess excellent electrical conductivity, efficient catalytic activity, and strong surface affinity toward LiPSs.Several catalysts including transition-metal oxides, [6][7][8] dichalcogenides [9][10][11][12][13] and their heterostructures [14,15] have been investigated to boost the interfacial reactions and improve conversion kinetics.Especially, the 2D heterostructures have attracted numerous attentions owing to the abundant active sites and strong interfacial electronic effects.Recently, the WS 2 -WO 3 heterostructure nanosheets with well-tuned composition and morphology were developed through a controlled sulfurization process of WO 3 nanoparticles. [16]In this designed catalyst, the strong adsorption toward LiPSs and good catalytic activity can be simultaneously achieved by the heterostructures interfaces, thus enhancing the sulfur reduction kinetics.Furthermore, various heterostructures, including MoO 3 /MoO 2 , [17] VO 2 /VN, [18] CoB/NBC [19] and CoSe 2 / Co 3 O 4 , [20] have also been developed to accelerate LiPSs conversion and combat the shuttling effect.Nevertheless, most of heterostructure electrocatalysts show poor conductivity and limited active sites because of the uncontrollable thickness.Most importantly, engineering ultrathin nonlayered heterostructure is difficult due to lack of anisotropic growth force. [21][24][25] In consequence, it is still challenging to synthesize heterostructure catalysts with ultrathin thickness and abundant exposed active sites to promote the strong adsorption of LiPSs and satisfactory electrocatalytic activity.
Here we present the design of nonlayered NiO/Ni 3 S 2 heterostructure nanosheets with ultrathin thickness as electrocatalyst to enhance the adsorption and conversion toward LiPSs.The NiO/Ni 3 S 2 were synthesized through a convenient topochemical conversion strategy by using Ni(OH) 2 ultrathin nanosheets as template.In the topotactic engineering, the layered Ni(OH) 2 nanosheet is one type of attractive template for preparing nonlayered ultrathin NiO/Ni 3 S 2 , which can provide a well-retained structural framing to keep the heterostructures at the thickness of atomic level.The as obtained NiO/Ni 3 S 2 ultrathin nanosheets provide sufficient active sites to form strong chemisorption for LiPSs, which can suppress the shuttling effect.Moreover, the NiO/Ni 3 S 2 as electrocatalyst effectively accelerates both the formation and decomposition of solid Li 2 S in discharge and charge processes, thereby preventing accumulation of LiPSs and improving the sulfur utilization.Thus, NiO/Ni 3 S 2 heterostructure with fast Li + transport and good conductivity is an ideal cathode to immobilize sulfur and unlock the high gravimetric capacity.These properties were confirmed by both experimental results including synchrotron-radiation X-ray absorption fine structure (XAFS), atomic force microscopy (AFM) in-situ Raman, and density functional theory (DFT) calculations.The cells with the NiO/Ni 3 S 2 electrocatalyst enable high areal capacity of 4.8 mAh cm −2 (6 mg S cm −2 ) under the low E/S (electrolyte/sulfur) ratio of 4.3 μL mg −1 .The strategy can also be extended to 2D Ni foil, which delivers a highly reversible specific capacity at 1 C over 300 cycles with a 0.018% decay per cycle.
Inspired by our previous report, the NiO/Ni 3 S 2 was prepared via a simple topochemical conversion by utilizing ultrathin Ni(OH) 2 nanosheets as template (Figure 1a). [26]In the typical process, layered Ni(OH) 2 template in situ grown on the surface of Ni foam by hydrothermal method (Figure S1, Supporting Information).Subsequent the template was annealed to give rise to the NiO followed by a controllable sulfurization.Unlike other synthesis methods, [27][28][29] the ultrathin layered template allows nondestructive topochemical conversion into nonlayered NiO/Ni 3 S 2 heterostructure with controlled thickness.Upon the inspection of Ni foam surface (Figure 1b) and crosssection (Figure S2, Supporting Information) images by scanning electron microscope (SEM), it clearly shows that the ligament surfaces are covered with the high-density arrays of NiO/Ni 3 S 2 ultrathin nanosheets.The NiO/Ni 3 S 2 catalyst (Figure 1c) possesses a high specific surface area and good conductivity, which can provide sufficient active sites for LiPSs adsorption and conversion.In addition, the thickness of NiO/Ni 3 S 2 nanosheets is measured to be ~1.7 nm by AFM (Figure 1d).The transmission electron microscopy (TEM) image in Figure 1e shows the 2D ultrathin nanosheets structure.As shown in Figure 1f, the explicit interface between NiO and Ni 3 S 2 can be observed, resulted from the partial sulfurization of NiO.This is further corroborated by selected area electron diffraction (SAED) patterns, in which well-resolved two sets of crystal lattices are corresponded to NiO and Ni 3 S 2 , respectively.Based on the high-resolution TEM (HRTEM) image of NiO/Ni 3 S 2 , the lattice spacing with 0.164 and 0.288 nm are indexed to the (300) and (110) planes of Ni 3 S 2 (Figure 1g) and 0.147 nm to the (220) plane of NiO (Figure S3, Supporting Information), respectively.High-angle annular dark-field scanning TEM (HAADF-STEM) and corresponding elemental mapping images show the S and O are homogeneously dispersed in NiO/Ni 3 S 2 nanosheets (Figure 1h), indicating that the coexistence of NiO and Ni 3 S 2 in the heterostructure.
X-ray diffraction (XRD) patterns were performed to analyze the details of phase and composition of the resulting materials.As shown in Figure 2a, the diffraction peaks for the materials after the first-step annealing at 37°, 43°and 62°can be ascribed to the (111), ( 200) and (220) planes of NiO (PDF #47-1049). [30]The appearance peaks after sulfurization at 31.1°, 49.7°and 55.1°are corresponded to (110), ( 113) and (122) planes of Ni 3 S 2 (PDF #44-1418), [31,32] which further proves the formation of NiO/Ni 3 S 2 heterostructure.It should be pointed out that the three strong peaks at 44.5°, 51.8°and 76.3°are resulted from the nickel foam substate.X-ray photoelectron spectroscopy (XPS) was further carried out to obtain the chemical composition as well as the binding states of the elements.The high-resolution Ni 2P spectrum (Figure 2b) shows two peaks at 855.7 and 873.5 eV, corresponding to Ni 2+ 2p 3/2 and Ni 2+ 2p 1/2 , and two peaks at 853.2 and 870.7 eV correspond to Ni 2p 3/2 and Ni 2p 1/2 of Ni 3 S 2 , respectively.In addition, there are two satellite peaks located at 860.9 and 878.9 eV.To precisely disclose the state of S element, S 2p highresolution XPS spectra was analyzed.As shown in Figure 2c, two peaks located at 161.5 and 162.7 eV are assigned to Ni 3 S 2 S 2p 3/2 and Ni 3 S 2 S 2p 1/2 , while the relatively weak peak at 168.1 eV is corresponding to oxidation of sulfur on the surface.Furthermore, the surface oxidation can also be reflected in O 1s (Figure 2d), where the peak at 531.4 eV represents the metal-oxygen bond, and the peak at 529.3 eV is attributed to sulfur-oxygen bond.
The deep insight into the coordination environment and electronic structure of NiO/Ni 3 S 2 is determinded by XAFS measurements.As shown in Figure 2e, the normalized XAS spectrum of NiO/Ni 3 S 2 contains distinctly different characteristic features to that of NiO and Ni 3 S 2 , which indicates the electronic structure change of the Ni.Comparing with NiO, the higher pre-edge peak intensity in NiO/Ni 3 S 2 is attributed to the increased dipole-allowed trasitions (1s -4p), which might occur through mixing of the 3d and 4p orbitals for the heterostructure. [33,34]Moreover, the white line intensity of NiO/Ni 3 S 2 is between NiO and Ni 3 S 2 , which reveals that the valence state of Ni in NiO/Ni 3 S 2 is between +1 and + 2. The evolution of the atomic coordination configuration of Ni was further studied by extended X-ray absorption fine structure (EXAFS) spectra in Figure 2f.The main peak at approximately 1.53 Å is attributed to the Ni-O contribution. [35]Further, the small peak at 1.93 Å is consistent with the Ni-S contribution. [36]The Fourier transformations of the EXAFS spectra of NiO/Ni 3 S 2 fit well with the strucutre optimized by DFT, indicating the heterosturcture of NiO/ Ni 3 S 2 (Figure S4, Supporting Information).The fitting results of the EXAFS spectra of NiO/Ni 3 S 2 and reference samples are summarized in Table S1, Supporting Information.
To investigate the electrocatalytic effect in Li-S batteries, Raman spectroscopy as an effective toll was employed to in situ monitor the conversions of LiPSs during charge and discharge processes. [10,23]Benefitting from the structure of Ni foam framework and the micron-sized ultrathin nanosheets, Raman signals of sulfur intermediates can be easily detected on the surface of catalysts (Figure S5, Supporting Information).From the real-time evolution of Raman images, all the longor short-chain LiPSs are clearly distinguished.Notably, compared with the conversion of S 8 into soluble LiPSs, the conversion of LiPSs into the Energy Environ.Mater.2023, 6, e12491 insoluble Li 2 S 2 /Li 2 S is more difficult and thus, represents the ratedetermining step in discharge process. [37]Encouragingly, the cell with NiO/Ni 3 S 2 possesses high catalytic activity, which can efficiently convert S 8 into S 2À 2 during discharge process and then transform into S 8 during charge process (Figure 3a).In contrast, the cell with only Ni 3 S 2 or NiO shows indistinguishable boundary of LiPSs during entire charge and discharge processes (Figure 3b,c), indicative of the accumulated long-chain LiPSs on the surface of catalyst owing to slow conversion kinetics.The slow conversion of LiPSs finally exacerbates the shutting effect and thus leads to rapid capacity fading.
Through in-situ Raman test, the conversions of long-, mid-, and short-chain LiPSs under the electrocatalytic effect can also be evaluated.As shown in Figure 3c, the open circuit voltage (OCV) before discharge shows four characteristic peaks, which correspond to antisymmetric bond-bending (150 cm −1 ), symmetric bond-bending (220 and 240 cm −1 ) and symmetric bond-stretching (475 cm −1 ) modes of the typical S 2À 8 anion. [38]During discharge process, the typical peaks of S 2À 8 gradually decline, while the peaks of S 2À 6 (400 cm −1 ) and S 2À 4 (455 and 535 cm −1 ) appear, respectively.With further discharge, the intensity of S 2À 2 (455 and 535 cm −1 ) peaks gradually increase, indicating that NiO/Ni 3 S 2 effectively promotes the LiPSs transformation, especially for the rate-determining steps. [39]uring charge processes, the peaks of S 2À 6 , S 2À 4 and S 2À 2 are completely oxidized to the state of S 8 , evidencing the high reversibility catalytic reaction by NiO/Ni 3 S 2 .These results collectively confirm that NiO/Ni 3 S 2 as electrocatalyst can simultaneously accelerate sulfur reduction reaction in the discharge process and enhance the oxidation kinetics of LiPSs in the charge process.In sharp contrast, neither S 2À 8 nor S 2À 7 (500 cm −1 ) can be completely converted into mid-or short chain LiPSs with pure NiO or Ni 3 S 2 catalyst during discharge process (Figure 3d).In terms of the cell with only Ni 3 S 2 or NiO, the Raman intensitys of S 2À 6 , S 2À 4 and S 2À 2 still remain strong at the end of charge process, indicating that the sluggish redox kinetic incapacitates for mitigating LiPSs shutting effect.Normally, the theoretical values of capacities and sulfur utilization of Li-S battery are closely related to Li 2 S nucleation/deposition process. [40,41]Thus, the Li 2 S precipitation tests were executed on different catalysts to further prove the superiority of NiO/Ni 3 S 2 toward the LiPSs transformation.The cell was first galvanostatically discharged to 2.06 V and followed by a potentiostatically discharged at 2.05 V.The Li 2 S deposition capacity was calculated based on Faraday's law.As shown in Figure 3g-i The cyclic voltammetry (CV) characteristics were committed to gain insight into the multiple redox reactions and the electrochemical enhancement mechanism during charge-discharge process.As shown in Figure 4a, the CV curves show two reduction peaks, which are ascribed to the transformation of Li 2 S 8 to the Li 2 S 4 and Li 2 S 4 into Li 2 S 8 , respectively.In the following anodic scan, one oxidation peak is corresponded to the conversion of Li 2 S into Li 2 S 8 . [42]It is noted that the cell with NiO/Ni 3 S 2 shows higher potential of cathodic peaks, lower potential of anodic peak and higher current density than NiO and Ni 3 S 2 , revealing the decreased polarization and the increased participation of active materials during redox reaction.Meanwhile, the reduction and oxidation peaks of NiO/Ni 3 S 2 /S overlap well after five cycles in CV curves (Figure S6, Supporting Information), which benefits from the good stability and reversibility of the cell.The symmetric cells with Li 2 S 8 electrode were investigated by CV measurement to further explore the catalytic ability of NiO/Ni 3 S 2 for LiPSs redox.As shown in Figure 4b, the CV curve of NiO/Ni 3 S 2 exhibits a higher current density than pure NiO and Ni 3 S 2 , demonstrating the significantly accelerated LiPSs conversion kinetics. [43]The discharge profiles of cells with different catalysts clearly exhibits two plateaus, corresponding to sulfur reduction of S 8 → Li 2 S 4 and Li 2 S 4 → Li 2 S, respectively.The one plateau for charge profiles is attributed to sulfur oxidation of Li 2 S → S 8 , which is well-consistent with CV curves in Figure 4a.For the NiO/Ni 3 S 2 /S electrode, the high peaks currents and small potential interval suggest a less electrochemical polarization compared to NiO/S and Ni 3 S 2 /S.The polarization voltage was increased during increasing sweep rate processes (Figure S7a-c, Supporting Information).The lithium-ion diffusion coefficient was estimated based on the Randles-Sevcik formula: where I p is on behalf of the peak current, n is the electron number in the redox reaction, A presents for the active area of the electrode, concentration, and v is the sweep rate.The Li þ diffusion coefficient is correlation with the I p values, which increases linearly with increasing the v 0:5 : [44] The NiO/Ni 3 S 2 /S shows a higher slope than NiO/S and Ni 3 S 2 /S (Figure S7d, Supporting Information), indicating a highest Li þ diffusion efficiency.In particular, the  4e.The NiO/Ni 3 S 2 /S performs a high initial discharge capacity of 920 mAh g −1 , which is higher than those of NiO/S (782 mAh g −1 ) and Ni 3 S 2 /S (527 mAh g −1 ).Meanwhile, the NiO/Ni 3 S 2 /S with excellent capacity cycle stability retains 764 mAh g −1 after 300 cycles, corresponding to an average capacity degradation of 0.05% per cycle.The stable cycling performance can be attributed to the effectiveness of NiO/Ni 3 S 2 catalyst for LiPSs adsorption and conversion, which fundamentally reduces the accumulation of In practical application, the performance with high sulfur mass loading and limited electrolyte are essential indicators of Li-S batteries.As shown in Figure 4f, the cell achieves 4.8 mAh cm −2 under a sulfur loading of 6 mg cm −2 and low E/S ratio of 4.3 μL mg −1 .Moreover, the average coulombic efficiency is 99% and the capacity retains 85% after 100 cycles.These results demonstrate that the ultrathin NiO/Ni 3 S 2 nanosheets can effectively suppress the shuttling effect and accomplish prominent cycling performance due to accelerated LiPSs conversion kinetics, even at high sulfur loading and low E/S ratio.The unique synthesis strategy for ultrathin NiO/Ni 3 S 2 heterostructure can also be realized on 2D Ni foil.The SEM images (Figure S10 S11, Supporting Information). [20]ased on the Hard Soft Acid Base theory, Ni 3 S 2 with soft basic S 2À 2 ions allows the existence of more covalent properties than that of NiO.Moreover, the interaction between the Ni atoms and S 2À 2 endows more valance electrons, which can promote the rapid electrons transport. [45]The bandgap of NiO/Ni 3 S 2 surface (0.01 eV) is much smaller than that of NiO (0.3 eV) and Ni 3 S 2 (0.04 eV), further indicating the stronger metallic properties in the NiO/Ni 3 S 2 heterostructure. [46]The binding energies between Li 2 S 4 and catalysts were calculated in Figure 5b, and the optimized adsorption configurations were exhibited in Figure S12, Supporting Information.Clearly, the adsorption energies of Li 2 S 4 on NiO and NiO/Ni 3 S 2 are higher than that on the Ni 3 S 2 , indicating that the NiO plays a major role in LiPSs adsorption.The Li ion diffusion properties on the surface of catalysts can reveal the catalytic capability of propelling the LiPSs redox process. [47]As shown in Figure 5c 5d), indicating that the NiO/ Ni 3 S 2 heterostructure can greatly increase the sulfur utilization and decrease the amount of dead Li 2 S. [48,49] In order to have a further insight into the catalytic effect on different catalysts, the molecular Energy Environ.Mater.2023, 6, e12491 models of Li 2 S 4 /Li 2 S and Li 2 S 4 /Li 2 S on catalysts were taken as the research objects in Figure 5e.It is worth noting that the bond length of S-S on NiO/Ni 3 S 2 surface is longest with 2.17 Å, indicating that NiO/ Ni 3 S 2 can effectively weaken the S-S bridged bond, and then accelerate the conversion from Li 2 S 4 into Li 2 S. The bond length of Li-S on the NiO/Ni 3 S 2 (2.41 Å) is longer than that of pristine Li 2 S (2.08 Å), NiO-Li 2 S (2.26 Å) and Ni 3 S 2 -Li 2 S (2.29 Å), which reveals that NiO/Ni 3 S 2 can facilitate the decomposition of Li 2 S. Charge density difference patterns between Li 2 S and catalysts were also considered, where the yellow regions attribute decrease of charge and the cyan regions denote charge accumulation.As shown in Figure 5f, there is more electron transfer between the S atom and NiO/Ni 3 S 2 , which can stretch and weaken the Li-S bond by forming Ni-S bond and thus, decrease the decomposition barrier of Li 2 S. [50,51] These results are in good agreement with the decomposition barrier calculations and the experiment results.
In conclusion, we constructed ultrathin nanosheets of NiO/Ni 3 S 2 heterostructure via a topochemical conversion of 2D Ni(OH) 2 template.The theoretical investigations confirmed that the NiO/Ni 3 S 2 heterostructure can promote the adsorption of LiPSs and accelerate both the formation and decomposition of solid Li 2 S through a balance between adsorption capability of NiO and catalytic activity of Ni 3 S 2 .Experimental results demonstrate that the cells with NiO/Ni 3 S 2 exhibit significant improvements in rate capability and cycling stability.Even under the high sulfur loading and lean electrolyte conditions, the Li-S batteries can deliver good cycling stability.This design can also be expended to 2D Ni foil, thus possessing potential for high gravimetric/ volumetric energy densities storage.This work provides an instructive strategy for designing ultrathin nonlayered heterostructure, which can be applied for energy storage and conversion devices.
, the capacity of Li 2 S deposition on NiO/Ni 3 S 2 shows a higher value of 584.4 mAh g −1 compared with NiO (554.2 mAh g −1 ) and Ni 3 S 2 (294 mAh g −1 ), indicating that the NiO/Ni 3 S 2 can effectively promote the conversion of LiPSs to Li 2 S.
is calculated to be 1.89 × 10 −20 cm 2 s −1 for the cell with NiO/Ni 3 S 2 / S, which is 2.8 and 9.8 times than those of NiO/S and Ni 3 S 2 /S, respectively.The superior Li + diffusion behavior for NiO/Ni 3 S 2 /S is attributed to the improvement adsorption and catalytic activity toward LiPSs, which can effectively reduce the electron transport energy barrier, decrease electrolyte viscosity, and inhibit insulator layer formation.As shown in Figure 4c, the charge-discharge profile of the cell with NiO/Ni 3 S 2 /S electrode shows the smaller polarization voltage gap of 207 mV, which is less than that of NiO (509 mV) and Ni 3 S 2 (500 mV) electrodes, demonstrating the fast redox reactions kinetics of NiO/Ni 3 S 2 /S electrode.Electrochemical impedance spectroscopy (EIS) of the Li-S standard batteries were tested to further verify the facilitated polysulfide conversion.In Nyquist plots (Figure S8, Supporting Information), the NiO/ Ni 3 S 2 /S exhibits a lower charge transfer resistance (R ct ) of ≈10.3 Ω than NiO/S (≈63.9Ω) and Ni 3 S 2 /S (≈137.9Ω), indicating that NiO/Ni 3 S 2 /S exhibited the efficient charge transfer and enhanced LiPSs redox kinetics.The NiO/Ni 3 S 2 ultrathin nanosheets with high catalytic activity provides opportunity to improve the Li-S batteries performance.The rate capabilities of cells with different catalysts were compared at various current densities from 0.1 to 2 C (1 C = 1675 mA g −1 ) to directly evaluate the effect of catalysts.The discharge capacities of NiO/Ni 3 S 2 /S electrodes deliver 1580, 1220, 1080, 930 and 640 mAh g −1 at 0.1, 0.2, 0.5, 1 and 2 C, respectively (Figure 4d).In contrast, the NiO/S and Ni 3 S 2 /S exhibit the poor rate performance, indicating that the NiO/Ni 3 S 2 catalyst can reduce the barriers of sulfur reduction reaction and hence enhance the LiPSs conversion compared with other catalysts.The cycle stability of NiO/Ni 3 S 2 /S, NiO/S and Ni 3 S 2 /S electrodes at 1 C are compared in Figure

Figure 3 .
Figure 3. a-c) In-situ Raman time-resolved spectra of NiO/S, and Ni 3 S 2 /S for (dis)charge processes, respectively.d-f) Selected Raman spectroscopy of NiO/S, and Ni 3 S 2 /S cathodes, respectively.The inserted red curves in a-c) are voltage-time profiles of NiO/S, and Ni 3 S 2 /S cathodes, respectively.Potentiostatic discharge profiles of Li 2 S 8 solution on g) NiO/Ni 3 S 2 , h) NiO and i) Ni 3 S 2 .
, Supporting Information) show that the NiO/Ni 3 S 2 ultrathin nanosheets densely coat on the surface of Ni foil.The cell with 2D Ni foil delivers a highly reversible specific capacity of 693 mAh g −1 at 1 C, and possesses excellent stability after 300 cycles with a 0.018% decay each cycle (Figure 4g).Moreover, the cells of NiO/Ni 3 S 2 /S on Ni foil achieve capacity of 542 mAh g −1 at 1 C under a sulfur loading of 3 mg cm −2 .These encouraging factors bring potential for the commercialization of Li-S batteries.The DFT calculations were used to help understand the roles of NiO and Ni 3 S 2 in their heterostructures.The partial density of stages (PDOS) is implemented to illustrate the d-band structure and bandgap change of NiO/Ni 3 S 2 .As shown in Figure 5a, the d-band center of NiO/Ni 3 S 2 is obviously closer to Fermi level compared with Ni 3 S 2 , which can induce stronger chemisorption toward LiPSs due to the less fill of antibonding states (Figure

Figure 4 .
Figure 4. a) CV curves of NiO/Ni 3 S 2 /S, Ni 3 S 2 /S, and NiO/S cathode at 0.1 mV s −1 from 1.7 to 2.8 V versus Li/Li + .b) CV curves of symmetric cells of NiO/ Ni 3 S 2 /S, Ni 3 S 2 /S, and NiO/S electrodes at 1 mV s −1 .c) Charge-discharge voltage profiles of NiO/Ni 3 S 2 /S, Ni 3 S 2 /S, and NiO/S electrodes at 1 C (1 C = 1675 mA g −1 ).d) Rate performances of the NiO/Ni 3 S 2 /S, Ni 3 S 2 /S, and NiO/S electrodes, e) Long-term cycling performances of NiO/Ni 3 S 2 /S, Ni 3 S 2 /S, and NiO/S electrodes with sulfur loading of 1 mg.f) Areal capacity of NiO/Ni 3 S 2 /S at 0.1 C. g) Long-term cycling performance of NiO/Ni 3 S 2 /S loading at Ni foil as electrode.h) Cycling performance of NiO/Ni 3 S 2 /S electrode with sulfur loading of 3 mg cm −2 .
, the Li ion diffusion barriers on Ni 3 S 2 (0.3 eV) and NiO/Ni 3 S 2 (0.37 eV) are lower than that of NiO (0.52 eV), which proves that the Ni 3 S 2 possesses superior catalytic activity.The detailed Li ion diffusion pathways on catalysts are shown in Figure S13, Supporting Information.Although NiO has the superior adsorption toward LiPSs, the lower conductivity and slowly Li ion diffusion behavior make it difficult to catalyze LiPSs conversion.Meanwhile, Ni 3 S 2 exhibits lowest Li ion diffusion barrier, whereas its weak adsorption ability results a large amount loss for the LiPSs.Combining the adsorption energy of Li 2 S 4 with the Li ion diffusion barriers results, it can be concluded that NiO/Ni 3 S 2 heterostructure achieves the balance of adsorption ability contributed by NiO and the catalytic capability derived from the Ni 3 S 2 for LiPSs conversion, thus improving the sulfur utilization.The decomposition energy barrier of Li 2 S on catalysts were also calculated to further reveal the catalytic kinetics in the charging process.The detailed dissociation processes on catalysts from the Li 2 S molecule into LiS cluster and a single Li ion are shown in Figure S14, Supporting Information.Compared to the decomposition barrier on NiO (0.57 eV) and Ni 3 S 2 (0.67 eV), the NiO/Ni 3 S 2 heterostructure shows the lowest decomposition barrier of 0.26 eV (Figure

Figure 5 .
Figure 5. a) Calculated PDOS of NiO/Ni 3 S 2 , NiO, and Ni 3 S 2 with aligned Fermi level.b) Binding energy of Li 2 S 4 on the NiO/Ni 3 S 2 , NiO, and Ni 3 S 2 , respectively.c) Lithium ion diffusion barriers and d) decomposition barriers of Li 2 S on the NiO/Ni 3 S 2 , NiO, and Ni 3 S 2 , respectively.e) The S-S bridged bond lengths of Li 2 S 4 and Li-S bridged bond lengths of Li 2 S on the NiO/Ni 3 S 2 , NiO, and Ni 3 S 2 , respectively.f) Side view for charge density difference of Li 2 S adsorption on the NiO/Ni 3 S 2 , NiO, and Ni 3 S 2 , respectively.The yellow and blue distributions correspond to charge accumulation and lose.The iso-surface is set to 0.0008 eV Å−3 .