Review Reports

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The paper is devoted to the preparation and characterization of double-doped  Na-transition metal complex oxides as cathode materials in sodium-ion batteries. This subject is important for practical applications.

The authors should justify why the double-doping is better than the single-one, the absence of this explanation is the main drawback of the paper.

p. 1, line 24: 2NFMTV should be instead of NFM.

p.2, lines 87-88: the expression “smaller ionic transfer energy band 125.4 mA h g-1 capacity at 0.1C” is not clear.

p. 3, line 107: it should be “sol-gel” instead of “Sol-get”.

p. 3, line 112: the chemical formula Fe2NO3 . 9 H2O is wrong.

The authors should reverse the sections 3.1.1 and 3.1.2, the XRD characterization should precede the SEM one.

The errors in cell parameters should be given (Fig. 3, Table 1).

It is not evident why Raman and FTIR experiments were necessary (section 3.1.4).

p. 14, line 447: D value is given only for 2NFMTV material, it would be advantageous to represent such values for other materials.

EIS results should be treated more carefully and deeply.

Author Response

Comment 1: The authors should justify why the double-doping is better than the single-one, the absence of this explanation is the main drawback of the paper.

Response 1: Thank you for the suggestion. We agree with this comment. Therefore, we added the information that can be found on line 91-109 as follows:

“A co-doping strategy is adopted to synthesize the  (NFMTV) where the NFM is doped with electrochemically inactive elements Ti and V by substituting a fraction of Fe to reduce the Jahn-Teller distortion. Another reason to substitute Fe is the instability of Fe⁴⁺ in oxide-ion configuration in ambient atmospheric condition. From the literature review, it is evident that while doping with a single element, the modifications in redox chemistry is somewhat restricted whereas an enhanced charge capacity, voltage stability, and total energy density can be achieved by creating a more tailored redox environment with dual dopants. This allows for the participation of diverse elements in redox processes. Because of these synergistic effects, battery performance may be optimized in ways that would be unattainable with single-element doping. Based on the study form previous research, it is observed that titanium (Ti) helps stabilizing the crystal structure for longer cycle life but it reduces the electronic and ionic conductivity to some extent. On the contrary, cathodes doped with vanadium (V) demonstrate promising results when operated at elevated voltages, leading to enhanced energy density and capacity retention enabling to function within a broader voltage range without experiencing rapid capacity degradation. By combining V and Ti together in NFM, it is apparent that the co-doped NFMTV would outperform the pristine NFM by improving conductivity, structural stability, and capacity retention.”

Comment 2: 1, line 24: 2NFMTV should be instead of NFM.

Response 2: Thank you for your observation. The 2NFMTV is added instead of NMF.

Comment 3: 2, lines 87-88: the expression “smaller ionic transfer energy band 125.4 mA h g^-1 capacity at 0.1C” is not clear.

Response 3: Thank you for the comment. The line is rewritten correctly. We added the information on line 86-88 as follows:

“Shen et al. developed a nano-necklace type  cathode structure that showed 125.4 mA h g-1 capacity at 0.1C and a 79% capacity retention after 300 cycles.”

Comment 4: 3, line 107: it should be “sol-gel” instead of “Sol-get”.

Response 4: Thank you for your observation. The typo is corrected.

Comment 5: 3, line 112: the chemical formula Fe2NO3.9 H2O is wrong.

Response 5: Thank you for your observation. The formula has been corrected . We added the information in line 129.

Comment 6: The authors should reverse the sections 3.1.1 and 3.1.2, the XRD characterization should precede the SEM one.

Response 6: Thank you for your suggestion. We have reversed the sections 3.1.1 and 3.1.2, where the XRD characterization precedes the SEM one as per your suggestion.

Comment 7: The errors in cell parameters should be given (Fig. 3, Table 1).

Response 7: The actual cell parameters are obtained after the Rietveld refinement. The goodness-of-fit parameters, R_wp and R_p are used to assess the quality of the fit between experimental data and a theoretical model. These parameters show us an estimation of accuracy and reliability of a fitted model. From R_wp and R_p values (should be <10), we can get an idea of error of the refinement and hence, the errors in the cell parameters.  The R_wp and R_p values associated with Rietveld analysis for lattice parameters are shown in Table 1.

Comment 8: It is not evident why Raman and FTIR experiments were necessary (section 3.1.4).

Response 8: Although Raman and FTIR are not necessary, these analyses were performed to bolster the statement and give better understanding why we chose 2NFMTV over all other samples.

Comment 9: 14, line 447: D value is given only for 2NFMTV material, it would be advantageous to represent such values for other materials.

Response 9: As we already got confirmed that 2NFMTV is the best material based on the material and electrochemical performance, therefore, we have run CV at different rates only for NFM and 2NFMTV.

Comment 10: EIS results should be treated more carefully and deeply.

Response 10: Thank you for your suggestion. We added more detailed explanation regarding EIS analysis on line 497-509 as well as supplementary document as follows:

 “The EIS for all the samples — pure NFM, 1NFMTV, 2NFMRV, and 3NFMTV — are shown in Figure S14-15, where it was evident that the 2NFMTV has the smallest charge transfer resistance, and the values are shown in Table 2. The diffusion value is calculated using following equation (2) and (3)

By using equation (2) and equation (3), the diffusion coefficients for all the samples are calculated and are shown in Table 2: 

It can be observed from Table 2 that 2NFMTV shows the superior conductivity based on the diffusion co-efficient value that matched well with values found in the CV analysis result.”

Reviewer 2 Report

Comments and Suggestions for Authors

The study focuses on a sodium-ion battery (SIB) cathode material, Na0.67Fe0.5Mn0.5O2 (NFM), which benefits from the natural abundance of iron (Fe) and manganese (Mn) and their high redox potentials. However, it faces structural instability due to Jahn-Teller distortion during cycling, causing performance issues. To address this, researchers synthesized a new cathode, Na0.6Fe0.5-2xMn0.5TixVxO2, by doping small amounts of titanium (Ti) and vanadium (V) into NFM. This doping mitigated Jahn-Teller distortion, improved structural stability, and enhanced electrochemical performance. The doped material showed better capacity retention and increased initial discharge capacity compared to pure NFM.

1 why do you choose two elements co-dopen? Why not try single element dopen? Why is the ratio of Ti to V 1: 1? It is worth trying different ratio of Ti to V.

2. In line 181, how can you get result that Ti+V wre well-perfused into bare NFM? SEM only tells the morphology, not elemental information.

3. in line 192, what is the reason for “primary particles became smaller in size and more compact in proportion to the increased ratio of V &amp; Ti doping”

4. what is theoretical capacity if NFM? In line 378, please explain the reason that the capacity of doped NFM is higher than pristine.

5. in fig 7 a and b, it is better to use the same color curve to represent the cycle number for charge/discharge.

6. In line 407, there are typos for C rate unit.

7. From line407 to 410, please reorganize your phrase to make sure it is easer to understand for readers.

8, there is a typo in line411. It should be : experienced”

9. in line 422, please explain the reaction behind the redox peak pairs.

10. in fig 8, please use mA/g as unit for current, not mA.

11 typo in line 464. It is fig8d, not fig 8b.

12, what does it mean by ”test EIS during charge and discharge?” it should be tested after charge or discharge finished for 1 st cycle.

13. line 175 to 177 should be deleted.

14. TEM was tested and showed in fig 1, it need to be explained in discussion part.

Author Response

Comments 1: why do you choose two elements co-dopent? Why not try single element dopen? Why is the ratio of Ti to V 1: 1? It is worth trying different ratio of Ti to V.

Responses 1: Thank you for the suggestion. We added the information on line 91-109 as follows:

“A co-doping strategy is adopted to synthesize the  (NFMTV) where the NFM is doped with electrochemically inactive elements Ti and V by substituting a fraction of Fe to reduce the Jahn-Teller distortion. Another reason to substitute Fe is the instability of Fe⁴⁺ in oxide-ion configuration in ambient atmospheric condition. From the literature review, it is evident that while doping with a single element, the modifications in redox chemistry is somewhat restricted whereas an enhanced charge capacity, voltage stability, and total energy density can be achieved by creating a more tailored redox environment with dual dopants. This allows for the participation of diverse elements in redox processes. Because of these synergistic effects, battery performance may be optimized in ways that would be unattainable with single-element doping. Based on the study form previous research, it is observed that titanium (Ti) helps stabilizing the crystal structure for longer cycle life but it reduces the electronic and ionic conductivity to some extent. On the contrary, cathodes doped with vanadium (V) demonstrates promising results when operated at elevated voltages, leading to enhanced energy density and capacity retention enabling to function within a broader voltage range without experiencing rapid capacity degradation. By combining V and Ti together in NFM, it is apparent that the co-doped NFMTV would outperform the pristine NFM by improving conductivity, structural stability, and capacity retention.”

Comments 2: In line 181, how can you get result that Ti+V wre well-perfused into bare NFM? SEM only tells the morphology, not elemental information.

Responses 2:  Thank you for the suggestion. Yes, SEM only tells the morphology, not elemental information. But in order to know whether the Ti+V are well-perfused or not, we did the EDS elemental mapping and by observing the mapping figures, we can conclude that the doping elements are well-distributed. We added the explanation on line 328-331 as follows:

“From the EDX elemental mapping images as well as in the quantitative values as shown in Figure 4(a)-(b), it was seen that all the elements – Mn, Fe, Ti, V and O were evenly dispersed homogeneously throughout the surface of the samples, validating Ti and V are doped well into the crystal structure.”

Comments 3: in line 192, what is the reason for “primary particles became smaller in size and more compact in proportion to the increased ratio of V &amp; Ti doping”

Responses 3:  Thank you for the suggestion. We added the information on line 305-308 as follows:

“The primary particles became smaller in size and more compact in proportion to the increased ratio of V & Ti doping suggesting a reduction in the crystal structure. Thus, it is postulated that the co-doping will enhance the electrochemical performance of the electrode by decreasing the route for Na-ion diffusion.”

Comments 4: what is theoretical capacity if NFM? In line 378, please explain the reason that the capacity of doped NFM is higher than pristine.

Responses 4:  The theoretical capacity of NFM is 260.7mAh g^-1. We have added in line 185 as the C rate.

Comments 5: in fig 7 a and b, it is better to use the same color curve to represent the cycle number for charge/discharge.

Responses 5:  We used the same color for same the same color curve to represent the cycle number for charge/discharge.

Comments 6: In line 407, there are typos for C rate unit.

Responses 6:  Thank you for your observation. The typo is corrected.

Comments 7: From line407 to 410, please reorganize your phrase to make sure it is easer to understand for readers.

Responses 7:  Thank you for your suggestion. The phrase is reorganized on line 428-431 as follows:

“The rate performance was performed for the pristine NFM and 2NFMTV at various current densities in the voltage range of 1.5-4.3 V as presented in Figure 7(d). While cycled at different C rates swiping from 0.1C (26 mAh g^-1) to 4C (1040 mAh g⁻¹), the 2NFMTV exhibited better rate capability than that of pristine NFM.”

Comments 8: there is a typo in line411. It should be : experienced”

Responses 8:  Thank you for your observation. The typo is corrected.

Comments 9: in line 422, please explain the reaction behind the redox peak pairs.

Responses 9:  Thank you for your suggestion. We have explained the reaction behind the redox peak pairs on line 448-451 as follows:

“As seen in the CV curves throughout the charge and discharge cycles, distinctive pairs of oxidation/reduction peaks emerged for pristine NFM and 2NFMTV, centered around 3.2 V/2.2 V and 4.3 V/3.1 V, respectively, corresponding to Mn³⁺/Mn⁴⁺ and Fe³⁺/Fe⁴⁺.”

Comments 10: in fig 8, please use mA/g as unit for current, not mA.

Responses 10:  Thank you for your observation. The mA is changed to mA/g is corrected. Please check Figure 8.

Comments 11: typo in line 464. It is fig8d, not fig 8b.

Responses 11:  Thank you for your observation. The typo is corrected.

Comments 12: what does it mean by ”test EIS during charge and discharge?” it should be tested after charge or discharge finished for 1 st cycle.

Responses 12:  Thank you for your observation. The phrase is corrected on line 482.

Comments 13: line 175 to 177 should be deleted.

Responses 13:  Thank you for your observation. The lines are deleted.

Comments 14: TEM was tested and showed in fig 1, it need to be explained in discussion part.

Responses 14:  Thank you for your suggestion. We have added the explanation for TEM analysis on line 308-311 as follows:

“The transmission electron microscopy (TEM) image, as shown in Figure 3 a) and 3b), also provides further evidence that the 2NFMTV particle has a more distinct hexagonal shape than that of pure NFM.”

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This manuscript reported a Ti and V co-doped P2-type NFM (Na_0.6Fe_0.5-2xMn_0.5Ti_xV_xO₂) as a cathode material for sodium-ion batteries. The authors demonstrated that 2% Ti/V co-doped NFM achieves an initial discharge capacity of 187 mA h g^–1 and a capacity retention of 47% after 150 cycles at 0.1C. The authors have systematically elaborated on the effects of Ti and V co-doping on the morphology, crystal structure, and chemical bonding of NFM, highlighting how they benefit the structural stability during cycling. The structural analysis is meticulous and comprehensive. Overall, I suggest this work to be published after minor revisions:

1. Compared with the previously reported single element doped NFM, what are the innovations/advantages of this Ti and V co-doping? A discussion on this aspect in the introduction section would be helpful for readers to understand the specific benefits of the proposed material. 

2.  In the introduction section, the description of the characterization techniques used (Lines 93-105) is better to be deleted or moved to the Materials and Methods section. 

3. The authors claimed that the Ti+V doping largely altered the Mn³⁺/Mn⁴⁺ ratio. Is there a correlation between the amount of doped Ti+V and the amount of increased Mn³⁺ content?

4. The statement in lines 356-357 regarding the “noticeable widening of the Raman spectra, particularly in the case of samples with Ti and V doped” appears to be inaccurate based on Figure 6a, where the 1NFMTV and 2NFMTV samples do not show noticeable widening. Please clarify or correct this observation. 

5. The authors suggested that Ti-V co-doping significantly suppresses the P2-OP4 phase transition, based on the charge curve for 2NFMTV in Figure 7a. However, in Figure S10, other Ti and V-doped NFMTV samples didn’t show the same suppression results. Can the authors comment on this?

6. In Figure 8a, the voltage gap between Mn³⁺/Mn⁴⁺ redox pairs (as well as the peak voltage gap between Fe³⁺/Fe⁴⁺redox pairs) enlarges for the 2NFMTV samples. Does this indicate increased polarization for the 2NFMTV sample? 

Author Response

Comments 1: Compared with the previously reported single element doped NFM, what are the innovations/advantages of this Ti and V co-doping? A discussion on this aspect in the introduction section would be helpful for readers to understand the specific benefits of the proposed material.

Responses 1: Thank you for the suggestion. We added the information on line 91-109 as follows:

“A co-doping strategy is adopted to synthesize the  (NFMTV) where the NFM is doped with electrochemically inactive elements Ti and V by substituting a fraction of Fe to reduce the Jahn-Teller distortion. Another reason to substitute Fe is the instability of Fe⁴⁺ in oxide-ion configuration in ambient atmospheric condition. From the literature review, it is evident that while doping with a single element, the modifications in redox chemistry is somewhat restricted whereas an enhanced charge capacity, voltage stability, and total energy density can be achieved by creating a more tailored redox environment with dual dopants. This allows for the participation of diverse elements in redox processes. Because of these synergistic effects, battery performance may be optimized in ways that would be unattainable with single-element doping. Based on the study form previous research, it is observed that titanium (Ti) helps stabilizing the crystal structure for longer cycle life but it reduces the electronic and ionic conductivity to some extent. On the contrary, cathodes doped with vanadium (V) demonstrates promising results when operated at elevated voltages, leading to enhanced energy density and capacity retention enabling to function within a broader voltage range without experiencing rapid capacity degradation. By combining V and Ti together in NFM, it is apparent that the co-doped NFMTV would outperform the pristine NFM by improving conductivity, structural stability, and capacity retention.”

Comments 2: In the introduction section, the description of the characterization techniques used (Lines 93-105) is better to be deleted or moved to the Materials and Methods section.

Responses 2: Thank you for the suggestion. The description of the characterization techniques used is moved to the Materials and Methods section.

Comments 3: The authors claimed that the Ti+V doping largely altered the Mn3+/Mn4+ ratio. Is there a correlation between the amount of doped Ti+V and the amount of increased Mn3+ content?

Responses 3: Thank you for the observation. Yes, there are correlation between the amount of doped Ti+V and the amount of Mn³⁺ content as we altered the redox number of Mn and hence, the amount of Mn³⁺ present in the material by altering the amount of Ti and V amount.

Comments 4: The statement in lines 356-357 regarding the “noticeable widening of the Raman spectra, particularly in the case of samples with Ti and V doped” appears to be inaccurate based on Figure 6a, where the 1NFMTV and 2NFMTV samples do not show noticeable widening. Please clarify or correct this observation.

Responses 4:  tahnk you for your suggestion. We modified the analysis result that can be found in line 374-378 as follows:

“The noticeable widening of the Raman spectra in case of 3NFMTV, may be attributed to either the presence of lattice disorder within the polycrystalline sample or the development of strain and the potential formation of sub-lattices within the matrix following the substitution of metals.”

Comments 5: The authors suggested that Ti-V co-doping significantly suppresses the P2-OP4 phase transition, based on the charge curve for 2NFMTV in Figure 7a. However, in Figure S10, other Ti and V-doped NFMTV samples didn’t show the same suppression results. Can the authors comment on this?

Responses 5: If we see carefully, the 1NFMTV and 2 NFMTV significantly suppressed the P2-OP4 phase transition as compared to pure NFM. 2NFMTV showed the superior performance among which we can see in the long-term charge/discharge data as well. But whenever the Ti/V percentage increased to 3% and onwards, then the phase transition issue again re-appeared, which can justify the reason why we are getting capacity decay in long-term cycles.

Comments 6: In Figure 8a, the voltage gap between Mn3+/Mn4+ redox pairs (as well as the peak voltage gap between Fe3+/Fe4+redox pairs) enlarges for the 2NFMTV samples. Does this indicate increased polarization for the 2NFMTV sample?

Responses 6: The peaks for 2NFMTV shifted to the right corresponding to the peaks for pure NFM. However, the peak voltage gap is not increased, and so we cannot say the polarization has increased for 2NFMTV samples.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

1.  I insist that errors in cell parameters should be given in Table 1. Moreover, 5 significant digits for 2NFMTV are too much for “a” parameter, and it has no physical sense, in case of pristine NFM there are only 2 digits. This table should be presented in a statistically correct manner, with correct number of significant digits for each parameter.

2.  I insist that the section 3.1.4 provides no insight in the investigation of these materials. It can be omitted. Lines 383-384: the sentence is not finished (Figure 6 (b) demonstrates that the FTIR spectra of NFM, 1NFMTV, 2NFMTV, and 383 3NFMTV samples).

3.  Concerning D value for 2NFMTV, from CV measurements it is equal to 8.4*10-12 cm2/s (line 472), but from EIS experiments it is equal to 8.04*10-6 cm2/s (Table 2). The authors should explain such a difference.

Author Response

Comments 1. I insist that errors in cell parameters should be given in Table 1. Moreover, 5 significant digits for 2NFMTV are too much for “a” parameter, and it has no physical sense, in case of pristine NFM there are only 2 digits. This table should be presented in a statistically correct manner, with correct number of significant digits for each parameter.

Response 1. Thank you for your suggestions. R_p, the residual error, and R_wp, the weighted residual error values are mentioned in Table 1. This is in alignment with other published papers where they mention R_p and R_wp error values obtained after the Rietveld Refinement. Also, we have altered the significant digits to 4 as the changes are very minute. We have also added the actual value for Pristine NFM that can be found on Table 1.

Comments 2. I insist that the section 3.1.4 provides no insight in the investigation of these materials. It can be omitted. Lines 383-384: the sentence is not finished (Figure 6 (b) demonstrates that the FTIR spectra of NFM, 1NFMTV, 2NFMTV, and 383 3NFMTV samples).

Response 2. Thank you for your suggestion. We have modified the sentences on line 383-385 as follows:

“Figure 6 (b) demonstrates the FTIR spectra of NFM, 1NFMTV, 2NFMTV, and 3NFMTV samples. The FTIR spectra of all samples’ bands span from 600 to 900 cm^-1 and correspond to the metal-oxygen bonds.”

Comments 3. Concerning D value for 2NFMTV, from CV measurements it is equal to 8.4*10-12 cm2/s (line 472), but from EIS experiments it is equal to 8.04*10-6 cm2/s (Table 2). The authors should explain.

Response 3. Thank you for the observation. This is a typo. The diffusion coefficient, D_Na⁺ value obtained from CV is 8.4×10^-6 cm²/s instead of 8.4×10^-12 cm²/s that is closely matched with D_Na⁺ value (8.04×10^-6 cm²/s) obtained from EIS analysis. This is now corrected on line 472.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

I agree to publish it. 

Author Response

Thank you very much for taking the time to review this manuscript. 

Round 3

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have treated well my remarks, and now the paper can be accepted in present form.