Strain-regulated Gibbs free energy enables reversible redox chemistry of chalcogenides for sodium ion batteries

Manipulating the reversible redox chemistry of transition metal dichalcogenides for energy storage often faces great challenges as it is difficult to regulate the discharged products directly. Herein we report that tensile-strained MoSe2 (TS-MoSe2) can act as a host to transfer its strain to corresponding discharged product Mo, thus contributing to the regulation of Gibbs free energy change (ΔG) and enabling a reversible sodium storage mechanism. The inherited strain results in lattice distortion of Mo, which adjusts the d-band center upshifted closer to the Fermi level to enhance the adsorbability of Na2Se, thereby leading to a decreased ΔG of the redox chemistry between Mo/Na2Se and MoSe2. Ex situ and in situ experiments revealed that, unlike the unstrained MoSe2, TS-MoSe2 shows a highly reversible sodium storage, along with an evidently improved reaction kinetics. This work sheds light on the study on electrochemical energy storage mechanism of other electrode materials.


Question 2.
The content of the 2-MI in the TS-MoSe2 is suggested to be quantified. Such as identifying the N content and then calculating the content of 2-MI.

Response：
We greatly appreciate your valuable comments. According to your professional suggestions, the content of the 2-MI in the TS-MoSe2 has been calculated based on the N content originating from CHN elemental analysis in Supplementary Table 2. Specifically, based on the chemical formula of the 2-MI, C4H6N2, the content of the 2-MI is calculated by the following equation, Where the ω2-MI and ωelement N are the mass fraction of the 2-MI and the element nitrogen, respectively. M2-MI and Melement N are the molar mass of the 2-MI (82 g/mol) and the element nitrogen (14 g/mol), respectively. And thus, the mass loading of 2-MI could be determined to be 4.10 wt.% ( The BET surface area of MoSe2 and TS-MoSe2 is suggested to be given. It is interesting that the specific capacity is largely enhanced. What is the inside reason for the enhancement?

Response：
We are very grateful for your valuable comments. Based on your professional suggestions, we used Brunauer-Emmett-Teller method to analyze the specific surface area and it can be seen that TS-MoSe2 was determined to be 17.68 m 2 g -1 , which is slightly higher than that of the unstrained MoSe2 (14.17 m 2 g -1 ) ( Supplementary Fig. 8).
This small difference between these two samples in BET surface area hardly leads to a nearly twofold increase in specific capacity. Thus, compared with MoSe2, the reversible storage of Na + and relatively higher Na + storage kinetics in TS-MoSe2 may be the major reason for its higher specific capacity.

Question 4.
Will the presence of 2-MI affect the formation of SEI layers, which is significant in the reactions.

Response：
Thanks for your good question. We conducted the linear sweep voltammetry (LSV) test through the three-electrode system to further confirm the stability of the 2-MI molecule. As shown in Supplementary Fig. 15, no visible reduction peak ascribed to the 2-MI molecule was found within the operating voltage window, indicating that the 2-MI is not involved in the formation of the SEI layer. (Please see pages 9 and 23 in the revised manuscript and Supplementary Fig. 15) Revised as follows: Corresponding revisions on page 9 in the revised manuscript: Furthermore, the stability of the 2-MI molecule was further confirmed by linear sweep voltammetry (LSV) curves, in which no visible reduction peak ascribed to the 2-MI molecule was found within the operating voltage window (Supplementary Fig. 15). Corresponding revisions on page 23 in the revised manuscript: The electrochemical stability of the 2-MI molecule was evaluated in a three-electrode configuration using glassy carbon electrode as the working electrode, platinum sheet as the counter electrode, and Ag/AgCl (in saturated KCl aqueous solution) as the reference electrode. The potential vs. Ag/AgCl was converted into potential vs. standard Na + /Na, under the assumption that the potential of the Ag/AgCl electrode was 3.326 V vs.

Question 5.
It is suggested that the authors place the right order of the figures, e.g in Figure 2 and 3.

Response：
Thank you for your suggestion, we have adjusted the layout of Fig. 2  This work reports the effort of adding organic molecules to make the Na/MoSe2 reaction reversible. The mechanism was not clearly described and it should not be published in its current form unless the following issues are satisfactorily addressed: Thank you very much for your constructive comments. These professional comments and suggestions are very helpful for us to improve the quality of our manuscript. We have revised the manuscript in accordance with your suggestions. Thanks for your valuable suggestions once again. Our responses to your questions are below: Question 1.
Most importantly, we expect from the theoretical part a comparison of the reversible reaction with the other unwanted irreversible reaction. Should the reversible reaction be preferred over the irreversible one where Mo stays inert?

Response：
We greatly appreciate your valuable comments. Following your suggestion, we added the calculation of the Gibbs free energy of the irreversible reaction (Mo + Na2Se → Mo + Se + 2Na + + 2e − ), ∆G = 3.76 eV) for the unstrained Mo in Supplementary Fig.   3, which is lower than that of the reversible one (∆G = 3.95 eV), indicating that the irreversible reaction may occur preferentially than the reversible one for the unstrained Mo. (Please see page 5 in the revised manuscript and Supplementary Fig. 2,3)

Revised as follows:
Corresponding revisions on page 5 in the revised manuscript: Furthermore, for the unstrained Mo, this ΔG value corresponding to the reversible reaction is higher than that of the irreversible reaction (Mo + Na2Se → Mo + Se + 2Na + + 2e − ) (Supplementary Fig. 3), indicating that the irreversible reaction may occur preferentially than the reversible one for the unstrained Mo.  Se-Na bond (Na2Se) upon discharging (Supplementary Fig. 23,24e). In the subsequent charging process, the Se-Na peak gradually disappears, while Se-Mo and Se-Se (TS-MoSe2) peaks become stronger. These observations also proved the excellent electrochemical reversibility of TS-MoSe2. In addition, the Se K-edge EXAFS spectra of TS-MoSe2 at D0.01 and C3.0 during the second and fifth cycles were also recorded, further confirming the reversible conversion of TS-MoSe2 in the subsequent cycles.
( Supplementary Fig. 26,27). Notably, neither elemental Se nor metallic Na was detected during the whole discharging and charging processes of TS-MoSe2, which is consistent with its reversible sodium storage process (MoSe2 + Na + ↔ Mo + Na2Se). Besides, due to the special "sandwich structure" of MoSe2, the intercalated Na + is located in the middle of the two layers of Se and is far away from Mo. Therefore, the Na-Mo bond can hardly be detected. Furthermore, ex-situ XPS was also conducted to probe the structural evolution during the electrochemical processes. Specifically, during the whole evolution processes of discharging and charging, ten voltages were selected to evaluate the structural transformation of the TS-MoSe2 electrode. As shown in the Mo 3d XPS spectra ( Fig.   3a), at the beginning of the discharging process (1.8 and 1. finally forms through the polyselenide Na2(Se)1+n (n > 1) during the discharging process ( Fig. 3c) 39 . Afterwards, in the following charging process, the peaks of both Mo 3d and Se 3d core levels can be fully recovered to their pristine state for TS-MoSe2, and in contrast, for unstrained MoSe2, the metallic Mo is always present and meanwhile the elemental Se is eventually generated ( Supplementary Fig. 17a,18a). These changes can be observed more visually in corresponding 2D mapping images of the Mo 3d and Se 3d XPS spectra ( Fig. 3b and Supplementary Fig. 17b,18b,19), which demonstrate that the strain engineering enables TS-MoSe2 to follow highly reversible sodium storage mechanism in the discharging and charging processes. Corresponding revisions on page 10 in the revised manuscript: Inspired by the positive influence that the strain engineering has achieved on the redox reaction by the DFT calculations, we first performed ex-situ XPS measurements to investigate the effect of the tensile strain on the sodium storage process. During the whole evolution processes of discharging and charging, ten voltages were selected to evaluate the structural transformation of the TS-MoSe2 electrode. As shown in the Mo 3d XPS spectra (Fig. 3a), at the beginning of the discharging process (1.8 and 1.5 V), two main characteristic peaks at 228.83 and 231.93 eV that are related to 3d5/2 and 3d3/2 of Mo 4+ in MoSe2 slightly shift towards the low binding energy, indicating the /2) appears and it can be assigned to metallic Mo 38 , suggesting that the NaxMoSe2 has partly transformed into the metallic Mo. At fully discharged state, the NaxMoSe2 completely disappears and only metallic Mo is detected. Correspondingly, the Se 3d peak at 54.5 eV first shifts to higher binding energy, and then restores to the original position, manifesting that Na2Se finally forms through the polyselenide Na2(Se)1+n (n > 1) during the discharging process ( Fig. 3c) 39 . Afterwards, in the following charging process, the peaks of both Mo 3d and Se 3d core levels can be fully recovered to their pristine state for TS-MoSe2, and in contrast, for unstrained MoSe2, the metallic Mo is always present and meanwhile the elemental Se is eventually generated ( Supplementary   Fig. 17a,18a). These changes can be observed more visually in corresponding 2D mapping images of the Mo 3d and Se 3d XPS spectra (Fig. 3b and Supplementary Fig. 17b,18b,19), which demonstrate that the strain engineering enables TS-MoSe2 to follow highly reversible sodium storage mechanism in the discharging and charging processes.  Ex-situ XPS characterizations: firstly, the battery was discharged and charged up to the required potential using a LAND workstation at a current density of 0.02 A g -1 .
Then, the battery was disassembled in a glove box to collect the electrode sheet.
Afterwards, the resulting electrode sheet was washed with dimethyl carbonate (DMC) to remove any residual salts. Finally, the tested electrode sheet was transported with a vacuum-transfer-module from the glove box to the XPS test system to avoid component changes when exposed to air. Furthermore, the electrode sheet was also etched with Ar + ion beam before the test to further avoid interference from surface SEI.

Question 3.
Some technical details should be fixed: The energy in Fig 1.

Question 4.
In addition, the mass loading of the active material is around 0.8-1.2 g cm -2 . The author should check it carefully. It should be mg cm -2 ? The loading is very low which will contribute to cycle stability and high rate. 30 wt% carbon black will help retard the side reaction.

Response：
Thanks for your valuable comments. First of all, we are very sorry for our carelessness. In this revision, we have corrected the unit of mass loading. Subsequently, in order to clarify the effects of mass loading and the content of carbon black on cycling and rate performance, we further tested cells with high loadings, low carbon content and simultaneously increasing loading and decreasing carbon content, respectively (Supplementary Fig. 33). Indeed, as you said, the cycle and rate performance of TS-MoSe2 and MoSe2 do show some degradation compared to before, which may be related to the electrolyte wettability and conductivity in the thick electrode.
Nevertheless, the overall performance of TS-MoSe2 is still significantly superior to that of MoSe2. (Please see page 17 in the revised manuscript and Supplementary 33)

Revised as follows:
Corresponding revisions on page 17 in the revised manuscript: Additionally, we also tested the cycling stability and rate performance of TS-MoSe2 electrode with increased loadings of the active materials. As shown in Supplementary   Fig. 33, only slight capacity reduction is observed. Besides, even with relatively high mass ratios of the active materials (the mass ratios of the active materials: carbon: binder are 7:2:1 and 8:1:1, respectively), TS-MoSe2 still exhibits good cycling and rate performance. conclude that "Mo participates in the entire electrochemical process and undergoes a reversible sodium storage reaction". Furthermore, it is not convincing that the presence of metallic Mo could be determined by one peak in Fig. 4c. I think C3.0 in Fig. 4C also contains metallic Mo, as it has an obvious peak shown below.

Response:
Thanks for your professional comments. We are so sorry for the unreasonable conclusion that was given in our original manuscript only from the XANES wiggle/oscillatory features of the post-edge region. Indeed, as you said, it seems that the oscillations of C3.0 and TS-MoSe2 are quite different in Fig. 4c, which may be due to the amorphous nature of the regenerated MoSe2 after full charging (C3.0) and poor signal-to-noise ratio. Furthermore, it is also not convincing to conclude the presence of metallic Mo from only one peak in Fig. 4c of the original manuscript.
In order to provide more sufficient evidence to prove the regeneration of MoSe2 after charging and the formation of metallic Mo during the discharge process, we further increased the mass loading of the active component of the tested electrode sheets to improve the signal-to-noise ratio according to the suggestion of the XAS testing engineer. And meanwhile, we also provided ex-situ XPS to further monitor the sodium storage process of TS-MoSe2.
Specifically, on the one hand, we increased the mass loading of the active component On the other hand, we further provided ex-situ XPS to confirm the formation of metallic Mo after discharging and regeneration of MoSe2 after charging. During the whole evolution processes of discharging and charging, ten voltages were selected to evaluate the structural transformation of the TS-MoSe2 electrode. As shown in the Mo 3d XPS spectra (Fig. 3a), at the beginning of the discharging process (  Corresponding revisions on page 10 in the revised manuscript: As shown in the Mo 3d XPS spectra (Fig. 3a), at the beginning of the discharging 230.53 eV (Mo 3d3/2) appears and it can be assigned to metallic Mo 38 , suggesting that the NaxMoSe2 has partly transformed into the metallic Mo. At fully discharged state, the NaxMoSe2 completely disappears and only metallic Mo is detected. Afterwards, in the following charging process, the peaks of Mo 3d core level can be fully recovered to their pristine state for TS-MoSe2. These changes can be observed more visually in corresponding 2D mapping images of the Mo 3d and XPS spectra (Fig. 3b), which demonstrate that the strain engineering enables TS-MoSe2 to follow highly reversible sodium storage mechanism in the discharging and charging processes. Photon energy (eV)

Fig. 3. Study on discharging and charging processes based on ex-situ XPS spectra. a-c ex-situ
Mo 3d XPS spectra (a) and corresponding mapping image(b), as well as Se 3d XPS spectra (c) of TS-MoSe2 during the initial discharging and charging processes.

Question 6.
The signal-to-noise ratio of XAFS seems very poor since there are many glitches at the white line peak of C3.0 in Fig. 4c. With such a poor signal-to-noise ratio, is the R space data credible? For example, the Mo-Mo peak is very low in D0.01, but the Mo-Mo peak in metallic Mo is very high. The highest peaks in D1.0 locate at ~1.2A and 5A, which is due to the poor quality of the data in my opinion. So, the k-space data should be provided.

Response:
Thank you very much for your professional advice. Indeed, as you said, the signalto-noise ratios at C3.0, D0.01, and D1.0 V (including C2.5 V for the first cycle, as well as the C3.0 and D0.01 V for the second cycle) are weaker than those at the other potentials. Then, by consulting with the engineer, we found that the poor signal-to-noise ratio may arise from the relatively low mass loading (0.8-1.2 mg cm -2 ) and the differences in structure, composition, and content of the species transformed under electrochemical conditions. To improve the signal-to-noise ratio of XAS data of these  in original manuscript. As expected, spectra with relatively stronger signals were obtained ( Supplementary Fig. 21,22,25). Specifically, as shown in Fig. 4c In addition, to further support the conclusion of reversible sodium storage of TS-MoSe2, we newly supplied ex-situ XPS to detect its structural evolution during discharging and charging processes. During the whole evolution processes of discharging and charging, ten voltages were selected to evaluate the structural transformation of the TS-MoSe2 electrode. As shown in the Mo 3d XPS spectra (Fig.   3a), at the beginning of the discharging process (1.8 and 1. finally forms through the polyselenide Na2(Se)1+n (n > 1) during the discharging process ( Fig. 3c) 39 . Afterwards, in the following charging process, the peaks of both Mo 3d and Se 3d core levels can be fully recovered to their pristine state for TS-MoSe2, and in contrast, for unstrained MoSe2, the metallic Mo is always present and meanwhile the elemental Se is eventually generated (Supplementary Fig. 17a,18a). These changes can be observed more visually in corresponding 2D mapping images of the Mo 3d and Se 3d XPS spectra ( Fig. 3b and Supplementary Fig. 17b,18b,19), which demonstrate that the strain engineering enables TS-MoSe2 to follow highly reversible sodium storage mechanism in the discharging and charging processes. (Please see page 10 in the revised manuscript, Fig. 3 and Supplementary Fig. 17,18,19) Revised as follows: Corresponding revisions on page 12 in the revised manuscript: In

Corresponding revisions on page 10 in the revised manuscript:
Inspired by the positive influence that the strain engineering has achieved on the redox reaction by the DFT calculations, we first performed ex-situ XPS measurements to investigate the effect of the tensile strain on the sodium storage processes. During the whole evolution processes of discharging and charging, ten voltages were selected to evaluate the structural transformation of the TS-MoSe2 electrode. As shown in the Mo 3d XPS spectra (Fig. 3a), at the beginning of the discharging process (1.8 and 1.5 /2) appears and it can be assigned to metallic Mo 38 , suggesting that the NaxMoSe2 has partly transformed into the metallic Mo. At fully discharged state, the NaxMoSe2 completely disappears and only metallic Mo is detected. Correspondingly, the Se 3d peak at 54.5 eV first shifts to higher binding energy, and then restores to the original position, manifesting that Na2Se finally forms through the polyselenide Na2(Se)1+n (n > 1) during the discharging process (Fig. 3c) 39 . Afterwards, in the following charging process, the peaks of both Mo 3d and Se 3d core levels can be fully recovered to their pristine state for TS-MoSe2, and in contrast, for unstrained MoSe2, the metallic Mo is always present and meanwhile the elemental Se is eventually generated ( Supplementary   Fig. 17a,18a). These changes can be observed more visually in corresponding 2D mapping images of the Mo 3d and Se 3d XPS spectra ( Fig. 3b and Supplementary Fig.   17b,18b,19), which demonstrate that the strain engineering enables TS-MoSe2 to follow highly reversible sodium storage mechanism in the discharging and charging processes.  Binding energy (eV) Supplementary Figure 18. Ex-situ Se 3d XPS spectra (a) and corresponding mapping image (b) of MoSe2 during the initial discharging and charging processes.

Supplementary Figure 19. Ex-situ
Se 3d XPS mapping image of TS-MoSe2 during the initial discharging and charging processes.
Finally, supposing every claim was well supported, I would not consider this material's potential for actual applications in sodium-ion batteries because it still suffers from the stress and strain issues common to all conversion-type electrodes. Highly desirable research in this field should address this issue first.

Response:
Indeed, as you said, the conversion-type electrodes always suffer from severe stress and strain issues, thus leading to structural failure, which limits their practical applications in sodium-ion batteries. While for MoSe2, its irreversible sodium storage eventually evolves into a Na-Se battery, which will suffer from the severe shuttle effect of polyselenides and poor structural stability, thus leading to rapid capacity attenuation.
This makes it more difficult to clearly distinguish the actual loss of MoSe2 electrode material from the irreversible reactions or from the stress and strain issues.
Based on this fact, we firstly focused on regulating the sodium storage mechanism of MoSe2 by constructing the out-plane tensile strain and in-plane compressive strain.
The results indicated that during the discharging process, our synthesized strained MoSe2 can transfer its strain to the discharged product Mo, thus enabling highly activity of Mo and accordingly achieving highly reversible sodium storage of MoSe2.
Meanwhile, we also supplemented the full cell performance of TS-MoSe2//Na3V2(PO4)2O2F in the revised manuscript ( Supplementary Fig. 35). It can be found that it delivers a performance comparable or superior to many reported full cell performances. (Please see page 17 in the revised manuscript and Supplementary

Fig. 35)
Thanks for your professional comments once again. According to your valuable suggestions, we will strive to address stress and strain issues based on the reversible sodium storage of MoSe2 to further improve its electrochemical performance in our future research.

Revised as follows:
Corresponding revisions on page 17 in the revised manuscript: In view of the superior sodium storage performance of TS-MoSe2 in the half cells, the Na-ion full cells were further assembled with homemade Na3V2(PO4)2O2F (NVPOF) as a cathode to preliminarily assess its practicability as an anode for SIBs ( Supplementary Fig. 34,35). As shown in Supplementary Fig. 35b,c, the full cells exhibit good cycling performance and the capacity can still maintain 434.9 mA h g -1 after 200 cycles at 0.2 A g −1 (based on the mass of the anode). The full cells also present superior rate capabilities (Supplementary Fig. 35d), in which about 70.2% of the capacity can be retained even when the current density increases by 50-folds from 0.1 to 5 A g −1 . The good rate capabilities endow the full cells with a specific energy of 108.6 Wh kg −1 at a power density of 19.0 W kg −1 , and even 74.1 Wh kg −1 at a power density of 648.5 W kg −1 (based on the total mass of the electrode materials), which are comparable or superior to those of many reported full cells (Supplementary Fig. 35e).  There are several published works on the topic of transition metal dichalcogenides for rechargeable alkali metal ion batteries. These materials tend to suffer from high first cycle loss, voltage hysteresis, considerable volume changes leading to capacity loss etc.

Supplementary
In addition, high cost of such materials is a major barrier for use in low-cost sodium ion batteries. In the present work authors describe synthesis of a MoSe2 type TMD, which under localized tensile deformation shows improved electrochemical performance as battery electrode over neat/pristine MoSe2. The work seem to be interesting but has some major flaws that need to be rectified before publication in any scientific journal.
Thank you very much for your constructive comments. These professional comments and suggestions are very helpful for us to improve the quality of our manuscript. We have revised the manuscript in accordance with your suggestions. Meanwhile, we also supplemented the full cell performance of TS-MoSe2//Na3V2(PO4)2O2F in the revised manuscript ( Supplementary Fig. 35). It can be found that it delivers a performance comparable or superior to many reported full cell performances. Thanks for your valuable suggestions once again. Our responses to your questions are below:  structural transformation of the TS-MoSe2 electrode. As shown in the Mo 3d XPS spectra (Fig. 3a), at the beginning of the discharging process (1.8 and 1.5 V), two main characteristic peaks at 228.83 and 231.93 eV that are related to 3d5/2 and 3d3/2 of Mo 4+ in MoSe2 slightly shift towards the low binding energy, indicating the formation of the NaxMoSe2 intermediate. With further discharging (1.0 and 0.4 V), a new component with lower binding energies at 227.43 (Mo 3d5/2) and 230.53 eV (Mo 3d3/2) appears and it can be assigned to metallic Mo 38 , suggesting that the NaxMoSe2 has partly transformed into the metallic Mo. At fully discharged state, the NaxMoSe2 completely disappears and only metallic Mo is detected. Correspondingly, the Se 3d peak at 54.5 eV first shifts to higher binding energy, and then restores to the original position, manifesting that Na2Se finally forms through the polyselenide Na2(Se)1+n (n > 1) during the discharging process (Fig. 3c) 39 . Afterwards, in the following charging process, the peaks of both Mo 3d and Se 3d core levels can be fully recovered to their pristine state for TS-MoSe2, and in contrast, for unstrained MoSe2, the metallic Mo is always present and meanwhile the elemental Se is eventually generated (Supplementary Fig. 17a,18a).
These changes can be observed more visually in corresponding 2D mapping images of the Mo 3d and Se 3d XPS spectra ( Fig. 3b and Supplementary Fig. 17b,18b,19), which demonstrate that the strain engineering enables TS-MoSe2 to follow highly reversible sodium storage mechanism in the discharging and charging processes. (Please see pages 10 and 22 in the revised manuscript, Fig. 3 and Supplementary Fig. 17, 18,

Revised as follows:
Corresponding revisions on page 11 in the revised manuscript: Furthermore, to exclude the influence of testing errors, we repeated the in-situ Raman testing and the experimental results are basically consistent (Supplementary Fig. 20 During the whole evolution processes of discharging and charging, ten voltages were selected to evaluate the structural transformation of the TS-MoSe2 electrode. As shown in the Mo 3d XPS spectra (Fig. 3a), at the beginning of the discharging process (1 Correspondingly, the Se 3d peak at 54.5 eV first shifts to higher binding energy, and then restores to the original position, manifesting that Na2Se finally forms through the polyselenide Na2(Se)1+n (n > 1) during the discharging process ( Fig. 3c) 39 . Afterwards, in the following charging process, the peaks of both Mo 3d and Se 3d core levels can be fully recovered to their pristine state for TS-MoSe2, and in contrast, for unstrained MoSe2, the metallic Mo is always present and meanwhile the elemental Se is eventually generated (Supplementary Fig. 17a,18a). These changes can be observed more visually in corresponding 2D mapping images of the Mo 3d and Se 3d XPS spectra ( Fig.   3b and Supplementary Fig. 17b,18b,19), which demonstrate that the strain engineering enables TS-MoSe2 to follow highly reversible sodium storage mechanism in the discharging and charging processes.

Corresponding revisions on page 22 in the revised manuscript:
Ex-situ XPS characterizations: firstly, the battery was discharged and charged up to the required potential using a LAND workstation at a current density of 0.02 A g -1 .
Then, the battery was disassembled in a glove box to collect the electrode sheet.
Afterwards, the resulting electrode sheet was washed with dimethyl carbonate (DMC) to remove any residual salts. Finally, the tested electrode sheet was transported with a vacuum-transfer-module from the glove box to the XPS test system to avoid component changes when exposed to air. Furthermore, the electrode sheet was also etched with Ar + Supplementary Figure 18. Ex-situ Se 3d XPS spectra (a) and corresponding mapping image (b) of MoSe2 during the initial discharging and charging processes. Binding energy (eV) ion beam before the test to further avoid interference from surface SEI.

Response：
We are sorry that our imprecise captions in Fig. 3 and 4 may have caused you some confusion. Actually, Fig. 3 and 4 mainly demonstrate that strain engineering enables TS-MoSe2 to achieve a reversible sodium storage mechanism during the charging and discharging processes rather than new mechanisms, and we have corrected the inappropriate captions and the corresponding subtitle in the revised manuscript.

Response：
We are grateful for your kind and valuable suggestions. We have marked the electrode mass involved in each electrochemical performance test in Fig. 5 and provided the corresponding areal capacity curves in Supplementary Fig. 32.
Additionally, we are so sorry that the description of electrochemical tests in the manuscript is incomplete. The number of specimens has been given in the experimental section, that is, 3 specimens were tested for each type of battery performance evaluation to ensure that they have almost identical results. Also, the galvanostatic chargedischarge curves of MoSe2 have been added in Supplementary Fig. 31. (Please see page 23 in the revised manuscript, Fig. 5 and Supplementary Fig. 31,32) Revised as follows: Corresponding revisions on page 23 in the revised manuscript: In order to effectively avoid errors introduced during the testing process, 3 specimens were tested for each type of battery performance evaluation to ensure that they have almost identical results.  There is barely any difference in the Na + diffusion coefficient in TS-MoSe2 over MoSe2 in Fig 6f. The unit has a factor of 10 -9 . In certain regions [voltage values], the calculated values of the diffusion coefficient overlap.

Response：
We appreciate the reviewer very much to notice this detail. Actually, there are still some differences in the Na + diffusion coefficients, and especially in the low de/intercalation states, the calculated DNa values of TS-MoSe2 are larger than those of MoSe2, although in some regions their DNa values almost overlap. This may be related to the different sodium storage processes that lead to varied intermediate phases.
Specifically, TS-MoSe2 experiences the in-/de-tercalation and conversion reactions (generally the former has a higher DNa due to weaker interlayer van der Waals forces 61,62 ), while for MoSe2, Se/Na2Se becomes the sole redox couple after the initial cycling that only occurs the conversion reaction (Se + 2Na + + 2e -↔ Na2Se). Thus, the

Response：
We greatly appreciate your constructive comments. The groundbreaking research of Singh on the application of TMD to Na + battery provides an alternative to the development on anode materials for sodium ion battery and the review article for green and sustainable Na + batteries by Tarascon and Larcher points the way for the subsequent development of Na + batteries, which have been cited and updated as ref. Overall, this is an interesting work with a fancy title [please consider removing terms like "strained-gene" from the title] that requires some additional experimental validation to support the conclusions about the superiority of TS-MoSe2 over MoSe2.
The work is obviously of some fundamental nature but of little use from practicality of TMD-based materials in batteries.

Response：
Thank you very much for your kind and professional suggestions. We have removed the terms of "strained-gene" from the title. In addition, we also newly added ex-situ XPS to further support the conclusion of reversible sodium storage of TS-MoSe2.
Specifically, during the whole evolution processes of discharging and charging, ten voltages were selected to evaluate the structural transformation of the TS-MoSe2 electrode. As shown in the Mo 3d XPS spectra (Fig. 3a), at the beginning of the discharging process (1.8 and 1.5 V), two main characteristic peaks at 228.83 and 231.93 eV that are related to 3d5/2 and 3d3/2 of Mo 4+ in MoSe2 slightly shift towards the low (Mo 3d5/2) and 230.53 eV (Mo 3d3/2) appears and it can be assigned to metallic Mo 38 , suggesting that the NaxMoSe2 has partly transformed into the metallic Mo. At fully discharged state, the NaxMoSe2 completely disappears and only metallic Mo is detected. Correspondingly, the Se 3d peak at 54.5 eV first shifts to higher binding energy, and then restores to the original position, manifesting that Na2Se finally forms through the polyselenide Na2(Se)1+n (n > 1) during the discharging process (Fig. 3c) 39 . Afterwards, in the following charging process, the peaks of both Mo 3d and Se 3d core levels can be fully recovered to their pristine state for TS-MoSe2, and in contrast, for unstrained MoSe2, the metallic Mo is always present and meanwhile the elemental Se is eventually generated (Supplementary Fig. 17a,18a). These changes can be observed more visually in corresponding 2D mapping images of the Mo 3d and Se 3d XPS spectra ( Fig. 3b and Supplementary Fig. 17b,18b,19), which demonstrate that the strain engineering enables TS-MoSe2 to follow highly reversible sodium storage mechanism in the discharging and charging processes. (Please see page 10 in the revised manuscript, Fig. 3 and Supplementary Fig. 17, 18, 19) Revised as follows: Corresponding revisions on page 10 in the revised manuscript: During the whole evolution processes of discharging and charging, ten voltages were selected to evaluate the structural transformation of the TS-MoSe2 electrode. As shown in the Mo 3d XPS spectra (Fig. 3a), at the beginning of the discharging process (1. 230.53 eV (Mo 3d3/2) appears and it can be assigned to metallic Mo 38 , suggesting that the NaxMoSe2 has partly transformed into the metallic Mo. At fully discharged state, the NaxMoSe2 completely disappears and only metallic Mo is detected. Correspondingly, the Se 3d peak at 54.5 eV first shifts to higher binding energy, and then restores to the original position, manifesting that Na2Se finally forms through the polyselenide Na2(Se)1+n (n > 1) during the discharging process (Fig. 3c) 39 . Afterwards, in the following charging process, the peaks of both Mo 3d and Se 3d core levels can be fully recovered to their pristine state for TS-MoSe2, and in contrast, for unstrained MoSe2, the metallic Mo is always present and meanwhile the elemental Se is eventually generated (Supplementary Fig. 17a,18a). These changes can be observed more visually in corresponding 2D mapping images of the Mo 3d and Se 3d XPS spectra ( Fig.   3b and Supplementary Fig. 17b,18b,19), which demonstrate that the strain engineering enables TS-MoSe2 to follow highly reversible sodium storage mechanism in the discharging and charging processes.  Binding energy (eV)