Intercalation-type catalyst for non-aqueous room temperature sodium-sulfur batteries

Ambient-temperature sodium-sulfur (Na-S) batteries are potential attractive alternatives to lithium-ion batteries owing to their high theoretical specific energy of 1,274 Wh kg−1 based on the mass of Na2S and abundant sulfur resources. However, their practical viability is impeded by sodium polysulfide shuttling. Here, we report an intercalation-conversion hybrid positive electrode material by coupling the intercalation-type catalyst, MoTe2, with the conversion-type active material, sulfur. In addition, MoTe2 nanosheets vertically grown on graphene flakes offer abundant active catalytic sites, further boosting the catalytic activity for sulfur redox. When used as a composite positive electrode and assembled in a coin cell with excess Na, a discharge capacity of 1,081 mA h gs−1 based on the mass of S with a capacity fade rate of 0.05% per cycle over 350 cycles at 0.1 C rate in a voltage range of 0.8 to 2.8 V is realized under a high sulfur loading of 3.5 mg cm−2 and a lean electrolyte condition with an electrolyte-to-sulfur ratio of 7 μL mg−1. A fundamental understanding of the electrocatalysis of MoTe2 is further revealed by in-situ synchrotron-based operando X-ray diffraction and ex-situ time-of-flight secondary ion mass spectrometry.


NCOMMS-22-50782 RESPONSE TO REVIEWERS' COMMENTS REVIEWER 1
General Comment: He et al reported an intercalation-conversion hybrid strategy for highperformance sodium-sulfur batteries where MoTe2 nanosheets-graphene composites were employed as an effective sulfur host with fast sodium-ion intercalation-deintercalation and catalytic activity.Impressively, the hybrid cathode exhibits a high capacity and excellent cycling performance under high sulfur loading and lean electrolyte condition, which are quite challenging for Na-S batteries.Such practical performance could be a benchmark for recent RT Na-S fields.I have some comments.

Response to General Comment:
We appreciate the reviewer very much for the positive feedback and valuable comments/suggestions, which were very useful for us to improve our manuscript.

Comment 1: From the authors' viewpoint, what applications should RT Na-S batteries be applied for in the future? What are the advantages of Na-S batteries over Li-S batteries besides the cost?
Response to Comment 1: We thank the reviewer for this comment.In our viewpoint, RT-Na/S batteries would be promising candidates for grid-scale energy-storage in the conceivable future.In addition, Na-S batteries with rich resources of both sodium and sulfur have also been considered as a supplement to Li-S batteries.In contrast to the limited resources (0.0018 wt% in the earth's crust) of Li, Na is more abundant (> 2.5 wt% in the earth's crust) [InfoMat, 2022, 4(5): e12291].
Comment 2: Some important references for hybrid strategies in Li-S batteries are missing, like ACS Energy Lett. 2018, 3, 3, 568-573, Nat Energy 4, 374-382 (2019).It is difficult for me to find the electrolyte used in this work based on the only information "were tested in the localized high-concentration electrolyte (LHCE) as reported in our previous work".Please add related references.

Response to Comment 2:
We thank the reviewer for the suggestions.The relevant work has now been added as new references 13 and 14 on page 2 in the revised manuscript.The LHCE work is cited as reference 22 on page 3 in the revised manuscript.
13. Chung, S., Luo, L., Manthiram A. TiS2-polysulfide hybrid cathode with high sulfur loading and low electrolyte consumption for lithium-sulfur batteries.ACS Energy Lett.In LHCE, the sulfur redox process is found to change from the conventional dissolutionprecipitation chemistry to a quasi-solid-state reaction.The quasi-solid-state reaction suppresses NaPS shuttling and ensures fast reaction kinetics.Meanwhile, LHCE promotes the formation of a stable SEI on Na metal, effectively prohibiting dendritic Na growth.However, there is still a challenge of sluggish kinetics when using LHCE.In this work, by introducing the intercalation-type catalyst, such issue has been overcome.

Comment 3:
Is there any visual evidence for the suppressed shuttling effect besides Fig. 3e?
Response to Comment 3: We thank the reviewer for this comment.Visual adsorption experiments were carried out in Na2S6 solution to evaluate the chemical anchoring ability of MTG toward polysulfides.As shown in Figure R3 below, the polysulfide solution containing MTG is completely transparent after adsorption.In contrast, the polysulfide solutions containing C remain light-yellow after adsorption.This indicates the much stronger absorptivity of MTG toward polysulfides than that of C. When pure MoTe2 (MT) is used as a sulfur host, it can also be used as a polysulfide absorber.However, the electronic conductivity of MoTe2 is not as high as graphene.Therefore, using pure MoTe2 may not be as beneficial to improve the sulfur redox kinetics.The strong coupling of graphene sheets with MoTe2 nanosheets enhances both ionic and electronic transport simultaneously, which is important to achieve fast sulfur redox.Figure R3 below has been added as Figure S3 in the revised supporting information.We have included the corresponding discussion on page 9 in the revised manuscript.

Comment 4: Why the authors choose MoTe2, not MoS2 which is cheaper?
Response to Comment 4: We thank the reviewer for this comment.As one of the many transition-metal dichalcogenides (TMDs), MoTe2 possesses a larger van der Waals interlayer distance (0.392nm) than MoS2 (0.347nm).The large interlayer space can easily accommodate the large Na-ions, resulting in a better TMD structural stability during the cycling process.Moreover, the electronic conductivity of MoTe2 is 1.8 S cm -1 , which is nine times higher than that of MoSe2 (0.2 S cm −1 ) [ phys.stat.sol.(b), 79: 713].Such results clearly demonstrate that MoTe2 can further promote the transport of electrons.We have included the corresponding discussion on page 3 in the revised manuscript.

Comment 5:
In Supplementary Table 1, the authors compared recent Na-S literature.I recommend using sulfur content in the cathode, not in composite to reflect the actual active material contained in the cathode.

Response to Comment 5:
We thank the reviewer for the suggestion, which helps to improve the quality of our manuscript.The sulfur contents in the cathodes have been revised to reflect the actual active material.

Response to Comment 6:
We thank the reviewer for the suggestion, which helps to improve the quality of our manuscript.The comparative cross-sectional morphologies of cycled Na in Na−S cells with MTG and C/S cathodes are shown below in Figure R4 to further demonstrate the advantage of MTG.As shown in Figure R4a below, the surface of cycled Na paired with MTG/S cathode is smooth without cracks.In contrast, the surface of the Na paired with C/S cathode is covered by a thick uneven layer with many cracks (Figure R4b), which may be attributed to the NaPS shuttling in C/S cathode.Figure R4 below has been added as Figure S6 in the revised supporting information.We have included the corresponding discussion on page 14 in the revised manuscript.Response to General Comment: We appreciate the reviewer very much for the positive comments and valuable comments/suggestions, which are very useful for us to improve our manuscript.We feel sorry that we did not state so clearly the novelty of our manuscript, Here, we would like to clearly state the significant novelty and the corresponding description has also been now included on page 2 in the revised manuscript.
To address the challenges of Na-S batteries, intensive research efforts, such as the combination of conductive carbon and sulfur, modification of separators with carbon, and development of sulfide cathodes have been pursued.In these systems, decent electrochemical performance has been illustrated with low sulfur loading (< 2 mg cm -2 ).However, recently reported work is mainly based on carbonate electrolytes, where a flooded electrolyte amount is required to fully wet the glass fiber separator.In this regard, it is still difficult to achieve high-performance Na-S batteries under rigorous conditions, including high active material loading and lean electrolyte, which is of crucial importance for practical Na-S batteries.A localized high-concentration electrolyte (LHCE) developed in our previous work enable a stable Na-S battery under a decent amount of electrolyte condition.However, the capacity is only 675 mA h g -1 after 300 cycles with a low sulfur loading of 2 mg cm -2 at a low rate of 0.1 C, which is far from the theoretical capacity of 1,675 mA h g -1 of Na-S batteries.These results indicate that the utilization of sulfur and the kinetics of the redox need to be further improved.
Herein, for the first time, we introduce a new concept of intercalation-conversion hybrid cathode to dramatically promote the kinetics of sulfur redox and improve the utilization of sulfur by coupling the intercalation-type catalyst MoTe2 with the conversion-type active material sulfur in Na-S batteries.In addition, MoTe2 nanosheets vertically grown on graphene flakes offer abundant active catalytic sites, further boosting catalytic activity for sulfur redox.
With the intercalation-conversion strategy and well-designed structure, a high capacity of 1,081 mA h g -1 with a capacity fading rate of only 0.05% per cycle over 350 is realized under high sulfur loading and lean electrolyte condition.In addition, the MTG/S can still deliver a discharge capacity of 637 mA h g -1 even over an impressive 900 cycles, corresponding to a capacity decay rate of 0.02% per cycle, which is the longest lifespan demonstrated to date.
The fundamental understanding of the electrocatalysis of MoTe2 is further revealed by in-situ synchrotron-based operando energy dispersive X-ray diffraction and ex-situ time-of-flight secondary ion mass spectrometry, which provides new insights and opportunities to develop advanced Na-S batteries with highly efficient electrocatalyst for sulfur conversion.
The concept of intercalation-conversion strategy provides a new route for designing novel sulfur cathode for Na-S batteries.Such a concept guides further exploration of promising sulfur cathodes with intercalation-type catalysts.These catalysts will enable high-energy cathodes for developing next-generation Na-S batteries as well as Li-S batteries.
Also, the fundamental understanding of MTG in Na-S batteries and the deep discussion on the electrochemical results collected under practically necessary conditions in this manuscript can provide new insights and opportunities to develop practical Na-S batteries.We believe that our work can motivate the rechargeable battery community to broaden their interest and benefit the energy storage area.

Comment 1: What about pure MoTe2 as sulfur host? Will it work as the polysulphide absorber such nicely without graphene too?
Response to Comment 1: We thank the reviewer for the comment.As shown in Figure R3 below, when pure MoTe2 is used as a sulfur host, it can also be used as a polysulfide absorber.However, the electronic conductivity of MoTe2 is not as high as graphene.Therefore, using pure MoTe2 may not be as beneficial to improve the sulfur redox kinetics.The strong coupling of graphene sheets with MoTe2 nanosheets enhances both ionic and electronic conductivities simultaneously, which is important to achieve fast sulfur redox.Figure R3 has been added as Figure S3 in the revised supporting information.We have included the corresponding discussion on page 10 in the revised manuscript.

Comment 4: Usually cathodes are prepared with pvdf binder in case of metal-sulfur batteries. Is there any particular reason to use CMC as binder for MoTe2 instead of pvdf?
What will be the performance with pvdf compared to cmc?
Response to Comment 4: We thank the reviewer for raising this important question.The main reason why CMC was used as a binder for the MTG/S electrode is that the Na ions present in the CMC could potentially provide some ionic conductivity, which can greatly help improve the reversible capacity of Na-S batteries.In addition, compared with PVDF, the electrode with CMC as binder shows improved electrochemical performance, which has been proved in previous work [Nat.Commun.2018, 9(1): 3870].We have included the corresponding discussion on page 22 in the revised manuscript.
highly efficient electrocatalysts for sulfur conversion.
The concept of intercalation-conversion strategy provides a new route for designing novel sulfur cathode for Na-S batteries.Such a concept guides further exploration of promising sulfur cathodes with intercalation-type catalysts.These catalysts will enable high-energy cathodes for developing next-generation Na-S batteries as well as Li-S batteries.
Also, the fundamental understanding of MTG in Na-S batteries and the deep discussion on the electrochemical results collected under practically necessary conditions in this manuscript can provide new insights and opportunities to develop high-performance Na-S batteries.We believe our work can motivate the rechargeable battery community to broaden their interest and benefit the energy storage area.

Comment 1:
The writing needs significant improvement.The language feels stiff, even for a scientific report, making it extremely difficult to read.

Response to Comment 1:
We thank the reviewer for the suggestion, which helps to improve the quality of our manuscript.As suggested, the language has been carefully improved in the revised manuscript.
Comment 2: Looking at their results, the sulfur utilization and capacity fade look much improved, but given the mass difference between carbon and MoTe2, I would like to see a comparison of the gravimetric energy density for the total mass of electrode material, not just active sulfur material.

Response to Comment 2:
We thank the reviewer for the comment.The content of the sulfur in each cathode is 70%.Here is the detailed electrode preparation: The MTG or Ketjen Black carbon was mixed with an appropriate amount of sulfur, and the mixture was heated at 155 °C for 12 h in a sealed vial under an Ar atmosphere.Then, the asobtained product was heated at 200 o C for 30 min in a flowing Ar atmosphere in a tube furnace to remove the redundant sulfur outside of the MTG or Ketjen Black carbon.The sulfur content in the composite was calculated to be ~ 87 wt.%.
We have added the cycling performance based on the total mass of electrode materials, which is marked with a unit of mAh g -1 e, as Figure S4 in the revised supporting information.The subscript e in mAh g -1 e means electrode.

Comment 3:
The authors conclude that their work "pave[s] the way for the practical application of Na-S batteries," but I question the use of Mo and Te as practical materials.This brings into question the relevance of this study.

Response to Comment 3:
We thank the reviewer for the comment.In this work, the MoTe2 was only selected as a model system to prove the concept of the intercalation-conversion hybrid cathode to promote the kinetics of sulfur redox and improve the utilization of sulfur for Na-S batteries.As demonstrated by the electrochemical performance, the concept of intercalation-conversion hybrid cathode was well proved, which can pave the way for the practical application of Na-S batteries.
To address the concern on the practical issue, we have revised all the sentences on practical application.
Comment 4: Delineation of the sulfur transformation and interfacial chemistry might be out of order.Figure 4 is showing sulfur distribution after 100 cycles without talking about the cycling performance first.

Response to Comment 4:
We thank the reviewer for the valuable suggestion.We moved the " Delineation of the sulfur transformation and interfacial chemistry" after the "Electrochemical performance discussion."

Comment 5:
The voltage window is large.Can the authors comment on the practicality of a 0.8V-2.8Vwindow?
Response to Comment 5: We thank the reviewer for the valuable suggestion.In our work, even in the range of 1.2 -2.8V, the MTG/S can still deliver a capacity of ~700 mAh g -1 , which can be employed in many systems.Also, the voltage can be increased by a series connection.
To address the concern on the practical issue, we have revised all the sentences on practical application.Response to Comment 7: We thank the reviewer for the valuable suggestion.We have deleted such a sentence to clear up any confusion.
Response to Comment 8: We thank the reviewer for the suggestion, which helps to improve the quality of our manuscript.According to the reference [Chemical Engineering Journal 452 (2023): 139111], as a two-dimensional family member of molybdenum-based materials, molybdenum telluride (MoTe2) has been considered a promising electrode material for sodium-ion storage because of much wider interlayer distance of 0.699 nm, which is larger than that in traditional graphite (0.335 nm) and MoS2 (0.615 nm) as well as MoSe2 (0.646 nm).
This corresponding part has been modified on page 3 in the revised manuscript to maintain the consistency in the article.Additionally, a new cathodic peak at 0.89 V and an anodic peak at 1.06 V appear in the CV curve of MTG/S cathode, which are related, respectively, to the sodiation of MoTe2 to NaxMoTe2 and desodiation of NaxMoTe2 to MoTe2.

Figure R3 .
Figure R3.Photograph of the sealed vials of a Na2S6/DME solution after contacting with C, pure MoTe2 (MT), and MTG.

Figure R4 |Comment 7 :
Figure R4 | Cross-sectional morphologies of cycled Na in Na−S cells with (a) MTG and (b) C/S cathodes.Comment 7: The compatibility of the electrolyte used in this work should also be further evaluated by Na-Cu or Al cells without the effect of sulfur cathodes.

Figure R5 | 2
Figure R5 | (a) CE of Na plating-stripping using Cu electrodes in LHCE at 1 mA cm -2 with an areal capacity of 1 mAh cm -2 and (b) the corresponding Na plating/stripping profile.

Figure R3 |
Figure R3 | Photograph of the sealed vials of a Na2S6/DME solution after contacting with C, pure MoTe2 (MT), and MTG.

Figure R7 .
Figure R7.SEM image of (a) MT/S and (b) MTG/S and their corresponding elemental mapping images of sulfur after 100 cycles.

Figure R1 |
Figure R1 | Cycling performances of MTG/S cathode and C/S cathode at 0.1C rate.Here, as

Comment 9 :
132 -Even when referring to previous work, you should quickly detail what electrolyte you are using.

Table R1 .
A comparative analysis of the Na-C cells with MTG/S and the state-of-the-art Na-S cells reported in the literature We thank the reviewer for the suggestion, which helps to improve the quality of our manuscript.As suggested, we have added the following more recent references in the revised manuscript.23.Zhang, E. et al.Single-Atom Yttrium Engineering Janus Electrode for Rechargeable Na-S Batteries.J. Am.Chem.Soc.144: 18995-19007 (2022).24.Zhou, Xue.et al.A High-Efficiency Mo2C Electrocatalyst Promoting the Polysulfide Redox Kinetics for Na-S Batteries.Adv.Mater.34: 2200479 (2022).25.Yang, H. et al.Architecting Freestanding Sulfur Cathodes for Superior Room-Temperature Na-S Batteries.Adv.Funct.Mater.31: 2102280 (2021).26.Fang, D. et al.Low-Coordinated Zn-N2 Sites as Bidirectional Atomic Catalysis for Room-Temperature Na-S Batteries.ACS Appl.Mater.Interfaces.15: 26650-26659 (2023).