In-Situ Photoelectron Spectroscopy Investigation of Sulfurization-Induced Sodiophilic Sites with Model Systems of α-sexithiophene and p-sexiphenyl

Round 1

Reviewer 1 Report

In this study, Prof. Chen and the co-authors characterized the interactions between sodium and thiophene/benzene oligomers-6T and 6P via in-situ XPS/UPS. Their observations are further confirmed by DFT calculation, that is: Na is preferentially to react with sulfur and its neighboring carbon in 6T while it is almost non-reactive with 6P-only charge transfer was observed. This study is practically meaningful since both solid-state and liquid batteries are applied to many thiophene and benzene derivatives for either anode protection, artificial SEI, or 3D metal host. To understand interactions between sodium and thiophene/benzene oligomers-instead of their polymers will supply a much clearer working mechanism. This study/report provides insight on the potential for further development of cheap and high-performance sodium metal batteries. I highly recommend this mechanism study and it is suggested to publish after carefully addressing the following issues:

(1)  The biggest concern is the structure of the work: the current storyline is to demonstrate the in-situ XPS/UPS results (with four configurations) first, then at the end to show the DFT calculation results. During the discussion, authors have to jump to the final DFT conclusion at each sub-session…

As a result, the current story looks like: this is what we observed in Figure 2…it is proved by Figure 6…; Figure 3 shows…. It is finally confirmed by Figure 6…., and at the end, shows Figure 6...

It is so confusing for readers, since they must look at the very end every time during each discussion.

If the structure can be reorganized, the logical flow for this study can be much improved: At first, authors can propose several potential interactions and host site between 6T and sodium, and do a DFT calculation, showing their adsorption energies differences. After that, the authors hypothesize which configurations are favorable between 6T with 1 Na atom, and 2 Na atoms, based on their adsorption energy. After that, authors designed a series of in-situ XPS experiments to prove this hypothesis. So, in general, show the DFT first, and following sessions can discuss the XPS/UPS to re-confirm the DFT and strengthen the initial hypothesis.

(2)  Introduction, Line 91-92. Authors can add a paragraph here to exaggerate the necessity for studying the interaction between thiophene/benzene and sodium. Some references regarding application of thiophene derivative polymers or small molecules such as P3HT or PEDOT on the anode side can be cited and discussed. Then the advantages for using 6T to do the study can be mentioned in here as well.

(3)  Materials and Methods, Line 113, full name of SAES need to be shown here.

(4)  Same paragraph as point3. The following information need to list:

(a)  Whether each S p1/2 and p3/2 pair peaks have fixed energy differences? (i.e. 1.16eV)

(b)  Whether different S components have the same FWHM?

(c)   How does each XPS spectra calibrate? Based C1s 248.6eV or not?

(d)  Was charge neutralizer utilized during XPS, if it does, what is the energy?

(5)  Results and Discussion, biggest concern regarding the in-situ XPS is whether the Na 1s peak had been tested, and are there any differences in the all four systems (6T/6P on Na; Na on 6T/6P)?

(6)  Session 3.1.1 thickness of 6T needs to be specified. So does 3.2.1 for 6P.

(7)  Session 3.1.1. authors might need to draw the ‘standing’ and ‘lying’ configurations for 6T as a Figure. Since 6T can vertically ‘stand’ on the substrate, either by only one end thiophene ring or by all six thiophene rings.

(8)  Figure 2(a), why do the Cc and Cs go to the higher binding energy after depositing more and more Sodium? Compared to the Cc for 0.3Na to the Cs-Na for 1.7Na, they almost have identical binding energy.

(9)  Figure 2(c) the Wf keeps decreasing from 2.84 to 2.36eV then increases to 2.78eV, can the authors explain why?

(10)       The compositional ratio for Figure 2 and 3 can be demonstrated.

(11)       Line 269-270, why is there a light peak broadening for C1s in 6P?

(12)       In the conclusion session, it is highly recommended for the authors to write some perspectives based on these observations. Especially, since thiophene oligomer is sodium reactive, it is suitable for which purpose in batteries? On the contrary, as a charge transfers favorable and sodium unreactive materials, how to apply benzene derivatives in SMA?

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

The paper reports a fundamental study to demonstrate the presence of chemical interaction between Na and organic material with S-functional groups. The study aims at adding some more knowledge for finding materials for stabilizing the Na batteries by using sulfur-containing protective materials for SMAs.

The study uses the classic XPS-UPS method that provides reliable information for the interactions at the Na-molecules interface. The observed interactions between Na and S-containing organic molecules are clearly demonstrated and the exact bonding configuration, supported by DFT calculations. The paper is well organized and deserves publishing in such dedicated issue since it is good demonstration that very basic start can add to selecting some important materials that will help for reducing the cost of using exclusively the expensive Li batteries.

 I suggest only that the authors add a comment whether this type of materials will also reduce  the very undesired dendrite formation. I also suggest that in Figure 2 the binding energy scale is inverted to be the same as in Figs. 2 and 3.

Author Response

Please see the attachment for detailed information.

Author Reply: Thank you for your valuable suggestions. Accordingly, the comments have been added to the conclusion section, and the binding energy scale has been inverted in the figures. To ensure clarity and ease of understanding, we have kept the scale consistent within each molecule model study. However, the scale in the 6T-Na interface figures is smaller than that in the 6P-Na interface figures in order to facilitate better observation of the peak compositions.

Author Response File: Author Response.pdf

Reviewer 3 Report

Figure 2a) and 199 and following lines:

Cc at d(Na)=1.7 nm is shifted to higher Binding energies and not constant for all thicknesses for Na, as claimed in the text. Which charge compensation was used? Why was no calibration applied? Since only shifts in relation to other peaks are mentioned it may be ok, This is a quasi insitu measurement, but it would be nice if the validity of the absolute Binding energies is properly mentioned/ discussed. In Fig. 5b and in the UPS/Valence band results the authors seem to be confident with the Energy calibration as well so some overall experimental details in short are suggested.

In the C1s spectrum C-Na2 peak is very low in Intensity in the three spectra and very difficult to justify the deconvolution in the quite broad C1s peak. The linewidth in all the spectra also increased, is there an explanation why the FWHM is not kept the same for the different spectra here as well like in the latter part?

Are there Na 1s  of other Na core spectra conducted and show evidence of the C-Na2 or S-Na1 and S-Na2. If not, maybe this should be mentioned and why this is not visible there.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The manuscript is revised and much improved. All the questions are well answered. It can be accepted in present form.