NC/Co@NC catalyst with hollow structure accelerates lithium-sulfur battery reaction kinetics

Lithium-sulfur (Li-S) batteries have gained significant interest due to their impressive energy density. Nonetheless, poor conductivity and shuttle effect hamper their further development. Here, we prepared NC/Co@NC catalyst materials by a self-templating method to improve the battery performance. The ZIF-8@ZIF-67@PDA was initially prepared by utilizing the favorable compatibility between ZIF and dopamine, and subsequently annealed to form NC/Co@NC materials. The NC/Co@NC catalyst can effectively adsorb lithium polysulfides (LiPSs), lower the reaction barrier and accelerate its conversion to lithium sulfide (Li2S). Moreover, the configuration of the hollow structure enhances the catalyst-electrolyte interface, which can maximize the catalytic ability of NC/Co@NC. The improved ion diffusion rates benefiting from the hollow structure can also be realized. With these combined effects, Li-S cells incorporating the NC/Co@NC hollow catalysts achieved a high sulfur utilization and superior rate behaviour, that is 1176 and 470 mAh g−1 at 0.2 and 4C, respectively. Furthermore, the cells achieved an ultralow capacity degradation of merely 0.08% per cycle after 300 cycles at 1C, further confirming their potential.


Introduction
Li-S batteries have inspired substantial research enthusiasm because of their remarkable advantages such as high energy density and cost effectiveness.The theoretical energy density of 2600 wh/kg can be achieved through reactions between S and Li to generate LiPSs and finally Li 2 S [1].Nevertheless, there has been no significant progress in their commercialization.The low conductivity of S and Li 2 S and the dissolution of LiPSs during cycling compromise its advantages, making it challenging to compete with lithium-ion batteries in the market [2].To address these challenges, researchers and experts have made significant efforts.Initially, carbon materials were introduced into the lithium-sulfur battery system [3].With conductivity increased, the S utilization was optimized.However, the dissolution of LiPSs remained unresolved, resulting in rapid capacity decay as the reaction progressed.
Later, polar materials such as oxides and sulfides were incorporated into Li-S batteries, which greatly mitigated the shuttle effect.Soluble LiPSs could be firmly adsorbed on the polar materials, which greatly improved the electrochemical behaviour of the battery [4].However, slow reaction kinetics lead to the prolonged presence of LiPSs, significantly increasing the risk of dissolution.Subsequently, catalyst materials caught the attention of scientists [5].The catalysts with adsorptive properties not only adsorb soluble LiPSs, but also lower the energy barrier and promote its conversion to solid Li 2 S 2 /Li 2 S, thereby alleviating the shuttle effect.Furthermore, the catalyst can also promote the conversion of bulk Li 2 S with low conductive to LiPSs, enhancing material utilization and improving the cycling stability of the battery simultaneously.For example, Yao et al. developed an MCONCFs catalyst, consisting of manganese-cobalt oxide and carbon nanofibers, which accelerates the reaction kinetics of cells [6].Wu et al. engineered an electrocatalyst, which effectively promotes the deposition and dissolution of Li 2 S, bidirectional catalyzing redox reactions in lithium-sulfur batteries [7].Although significant progress has been made, catalyst materials with favourable adsorption and catalytic properties need to be continuously explored.
In this work, a hollow NC/Co@NC catalyst was elaborately designed by employing ZIF-8 and ZIF-67 as the template and cobalt source.Subsequently, the NC/Co@NC catalyst was coated onto the separator to effectively adsorb and catalyze LiPSs.The combination of conductive carbon with strong catalytic activity Co species, not only effectively addresses issues related to poor conductivity in batteries, but also suppresses the dissolution of LiPSs and accelerates their conversion.Additionally, the hollow structure provides a large specific surface area, leading to increased catalytically active sites, and simultaneously promotes ion diffusion.Consequently, the battery equipped with NC/Co@NC catalyst exhibited remarkable performance: a high sulfur utilization of 1176 mAh g -1 at 0.2C, favorable rate capability with 470 mAh g -1 at 4C, and impressive cycle stability, retaining 746 mAh g -1 even after 300 cycles at 1C.

Synthesis of ZIF-8
0.875 g zinc nitrate hexahydrate and 1.7 g 2-methylimidazole (2-MIM) were dissolved in 20 and 60 ml methanol, respectively.The two were mixed and stirred for 12 hours.

Synthesis of ZIF-8@ZIF-67
6 mmol cobalt acetate and 1.97 g 2-MIM were dissolved in 30 and 80 ml methanol, respectively.ZIF-8 was added to one of the solutions, and the mixture was mixed and standing for 24 hours.

Synthesis of NC/Co@NC
The obtained ZIF-8@ZIF-67@PDA was annealed at 900 °C under N 2 for 3 hours to achieve NC/Co@NC catalyst.

Preparation of the modified separator
80 wt% NC/Co@NC, 10 wt% super P, and 10 wt% PVDF were uniformly dispersed in NMP, and the obtained homogeneous slurry was coated on a separator and dried at 65 °C.

Results and discussion
The synthesis route is described as follows.Initially, ZIF-8 was chosen as the template for our subsequent work.Next, the ZIF-8@ZIF-67 material was formed due to the similar crystal lattice structures of ZIF-8 and ZIF-67.Subsequently, DA was coated onto ZIF-67 via simple stirring, without the use of a buffer solution or high-temperature treatment, resulting in the production of the ZIF-8@ZIF-67@PDA.The successful encapsulation of dopamine can be explained as follows.Dopamine interacts with ZIF-67, inducing the release of Co 2+ and 2-methylimidazole.The methanol solution of 2methylimidazole creates an alkaline environment, triggering the polymerization of dopamine.Importantly, Co 2+ further accelerates the polymerization of dopamine, significantly reducing the reaction time.Finally, the ZIF-8@ZIF-67@PDA was annealed at 900°C under nitrogen.The shrinkage of the ZIF-8@ZIF-67@PDA inevitably results in the movement of Co species, which are subsequently reduced to cobalt metal, while Zn elements evaporate in an environment exceeding 800°C.The generation of the hollow structure is due to the different carbonization temperatures of ZIF-8, ZIF-67 and PDA.Dopamine is initially carbonized, inevitably undergoing contraction.During this process, ZIF-67 and ZIF-8 provide structural support, preventing the inward collapse of the resulting carbon shell.Next, ZIF-67 undergoes a similar process during subsequent carbonization.Finally, ZIF-8 was subjected to an outward supporting force as it contracted at high temperature from the rigid shell generated from the carbonization of ZIF-67, preventing inward contraction of ZIF-8, and thereby forming the hollow structure.
Figure 1a is a scanning electron microscope (SEM) image of ZIF-8.The size is approximately 90 nm.After ZIF-67 growth, the particle size increased to approximately 140 nm, confirming the successful preparation of ZIF-8@ZIF-67 material (Figure 1b).The mutual growth or composite synthesis of ZIF-8 and ZIF-67 represents a widely adopted approach in the construction of porous materials, offering a multitude of advantages.Firstly, it allows precise control of the pore size by adjusting the proportion of ZIF-8 and ZIF-67, thus enabling the achievement of desired pore dimensions adaptable to various application requirements.Secondly, this approach significantly increases the specific surface area of hollow materials, improving adsorption and catalytic performance, which is particularly useful for gas separation and catalytic reactions.At the same time, ZIF-8 and ZIF-67 materials exhibit distinct properties and functionalities, and through the synthesis of composite structures, these properties can be fused to create multifunctional materials suitable for a wide range of applications.This method typically operates under relatively mild conditions and uses renewable organic precursors in the synthesis, helping to reduce environmental impact and align with principles of sustainability.In particular, the synthesis of ZIF-8@ZIF-67 structures offers researchers the opportunity to explore new fields such as nanotechnology, gas separation, drug delivery, and energy storage, thus enriching the research landscape in the fields of science and engineering.Next, there was no significant change in the size of the material after dopamine coating, but the particles were more tightly attached (Figure 1c).After annealing, the NC/Co@NC catalyst material was produced with an irregular morphology, which is attributed to the shrinkage of the material at high temperatures.As is shown in Figure 1d, the NC/Co@NC has a hollow structure, which can be confirmed by the opening structure.The NC/Co@NC has the following advantages: first, the carbon material formed by dopamine carbonization can greatly boost the conductivity of Li-S cells.Second, Co metal can not only effectively adsorb LiPSs, but also promote its bidirectional conversion, greatly accelerating the reaction.Third, the hollow structure can enhance the catalyst-electrolyte interface, and also promote Li + diffusion.Next, the catalytic properties of the NC/Co@NC catalysts were investigated.Figure 2a represents the cyclic voltammetry curves.Peak A represents the conversion of Li 2 S to S 8 , and Peaks B and C are the conversion of S 8 to LiPSs followed by Li 2 S. As the scan rate increases, there is a corresponding rise in peak current and the redox peaks shift to either side, respectively.It is noteworthy that there is a negligible shift in the peak from 0.2 to 0.25 mV s -1 , suggesting superior chemical stability.Figure 2b is the electrochemical impedance spectrum (EIS) of the battery with NC/Co@NC catalysts, which consists of two semicircles and an inclined straight line.Two semicircles in the high and middle frequency region can be attributed to the charge transfer impedance (Rct) and the dissolution of LiPSs, respectively.Rct measures the resistance to electron transfer at the interface between the electrode and electrolyte, which is a critical factor influencing the rate and efficiency of electrochemical reactions in the battery.After circuit fitting, the Rct is 15.44 Ω, which is relatively low compared to other reported results in the literature, indicating that the catalyst can improve battery conductivity and facilitate efficient charge transfer.Figure 2c illustrates the rate response of the NC/Co@NC modified batteries.The battery achieves specific capacities of 1176, 998, 842, 659, and 470 mAh/g at 0.2, 0.5, 1, 2, and 4 C, respectively.Moreover, a high specific capacity of 1045 mAh/g can still be obtained when returning to 0.2 C. Finally, the long-term performance of the cell was investigated (Figure 2d).A low capacity decay rate of 0.08% was achieved after cycling 300 cycles at 1C, proving the excellent cycling stability.

Conclusion
In the study, we designed a hollow NC/Co@NC catalyst material with high catalytic activity.The Co metal can facilitate bidirectional conversion between LiPSs and Li 2 S, enhancing the reaction kinetics, while the hollow structure exposes more catalytic sites.In addition, the incorporation of carbon material greatly improves the electrical conductivity.The battery with NC/Co@NC catalyst achieves outstanding electrochemical properties with 1176 mAh/g at 0.2C, 470 mAh/g at 4C, and a low capacity fading rate of 0.08% per cycle at 1C after 300 cycles.This study not only demonstrates the promising potential of NC/Co@NC in Li-S systems, but also emphasizes the critical role of innovative catalysts in shaping the future of energy storage systems.