High-voltage and dendrite-free zinc-iodine flow battery

Zn-I2 flow batteries, with a standard voltage of 1.29 V based on the redox potential gap between the Zn2+-negolyte (−0.76 vs. SHE) and I2-posolyte (0.53 vs. SHE), are gaining attention for their safety, sustainability, and environmental-friendliness. However, the significant growth of Zn dendrites and the formation of dead Zn generally prevent them from being cycled at high current density (>80 mA cm−2). In addition, the crossover of Zn2+ across cation-exchange-membrane also limits their cycle stability. Herein, we propose a chelated Zn(P2O7)26- (donated as Zn(PPi)26-) negolyte, which facilitates dendrite-free Zn plating and effectively prevents Zn2+ crossover. Remarkably, the utilization of chelated Zn(PPi)26- as a negolyte shifts the Zn2+/Zn plating/stripping potential to −1.08 V (vs. SHE), increasing cell voltage to 1.61 V. Such high voltage Zn-I2 flow battery shows a promising stability over 250 cycles at a high current density of 200 mA cm−2, and a high power density up to 606.5 mW cm−2.

Manuscript 463657 entitled "High Voltage and Dendrite Free Zinc Iodine Flow Battery" by Wang and colleagues reports on the development of a Zinc-pyrophosphate chelate and its use as negative electrolyte material in Zinc-iodine flow batteries.The approach is interesting and the paper is fairly well written, but I am not convinced by the claimed mechanism.The authors state that the chelate enables dendrite free plating of zinc.I suspect, however, that the zinc complex remains in solution and acts a "regular" dissolved redox active molecule and that the zinc plating that is observed is simply from uncomplexed zinc in solution.I can therefore not recommend publication of the article.If the authors can provide compelling evidence about the zinc plating behavior the manuscript could be reconsidered.Main concern: L156: With such a strong binding energy, how is the chelate going to fall apart and plate zinc effectively?This is the first hint that you are not plating zinc but reducing the Zn-complex.L175: Why would the plating/striping potential decrease that much?At some point during the zinc plating reaction the complex would have to fall apart forming free zinc which could then be plated.This may be somehow assisted by the pyrophosphate but it would still be Zn2+ + 2e---> Zn0.Fig3a: this does not at all look like a plating striping reaction.It looks much more like a "normal" CV of a redox active molecule in solution.L258: Upon discharge any Zn2+ released from the anode would immediately be chelated by the pyrophosphate?It seems as if this process should take some amount of time, as described in your synthesis section.Figure 5e: The intensity of plated Zinc is smaller at higher currents, this does not make any sense at all.Are you sure the "smooth" Zn you show in the SEM figure is not just uncomplexed Zn that is plated?L322: Please explain why the 4-charged chelate is attracted more strongly to the negative electrode than a Zn2+ cation or water?Please explain how the water activity is decreased?If there are less Zn2+ ions that strongly bind water around themselves, how is the water activity lower?
To convince me, please quantify how much Zn you plate from the Zn-chelate solution.
Below are some additional remarks: L40: the separation of energy and power components is largely lost with a plating/striping electrode like zinc L42: what structural features?L44: VRFB have developed far beyond demonstration purposes with 100's of MWh installed globally L52: corrosion of what?L55: Why is Zn-dendrite formation more pronounced at high currents?L57: Does the "dead Zn" dislodge and then flow through the cell?Does it clob the flowfields in any way?L60: Can you explain the "H2O induced corrosion"?Why is Zn2+ crossing into the positive side a problem, does Zn2+ react with the cathode? Figure 1: I think it would help the reader to have some visual representation of the structure included in the MS/NMR/Raman/FTIR data.L114: "meanwhile" not "in the meanwhile" L118: Why do the PO3 vibrations shift to higher wavenumbers?Can you explain what is going on here?L125: Why did you chose a molar ratio of 3:1 and not 2:1?L125: How did you determine the solubility?L138: Why do you compare to ZnBr2?Because of Zn-Br flow batteries?I think it would be good to spend 1-2 sentences explaining this.L148: Is this concentration dependent?L160: Please explain ESP with 1-2 sentences.L165: The molecule has a 6-charge but it is electrophilic?How does that work?Why do you reference Fig 2d here?L170: Can you elucidate why the LUMO of the complex is higher than the free Zn2+?L204: Why is it described as if surprising that Zn2+ goes through a cation exchange membrane?What else would you expect? Figure 4a: There are bumps in the blue curves at ca .,20mAh and 90 mAh.What is going on there? Figure 4f: Why is the charge polarization decreasing upon cycling?L224: Again, why is it presented as suprising that the negatively charged chelate does not go through the cation exchange membrane?L292: Coulombic interaction between the membrane and the electrolyte, why would this affect the dendrite formation?L294: The high nucleation overpotential indicates that there should be a ca.200 mV overpotential for plating Zn in the pyrophosphate solution?But you do not observe such a cell polarization?L323: to analyze not to analysis L335: Discussion --> Conclusion: How does your work guide future efforts?Please explain better the key learnings and how they can guide future work.
Reviewer #2 (Remarks to the Author): This manuscript presented a very nice work on negolyte development for zinc-iodine flow battery.Author's innovative refreshing chemistry by chelating K4P2O7 with Zn2+ has produced a negolyte that enabled a high-voltage and dendrite-free zinc-iodine flow battery, performing significantly better than conventional zinc-iodine flow battery in terms of working current density and areal capacity.Their effort and novelty are to be commended, which will have important impact for flow battery technology.I recommend the manuscript to be accepted and published in Nature Communications.Some minor comments are as following: 1.For Zn(PPi)26-based negolyte, why was ZnCl2 employed, not ZnBr2 (consistent with the results for Zn(H2O)62+)?2. For the permeability measurement of Zn2+, the right compartment was filled with 20 mL of 0.4 M KCl, while in Supplementary Fig. 11's caption, saturated zincon monosodium salt is added in the reference cell, is the zincon monosodium salt added into the right compartment prior to the permeability measurement?3. How about the stability of Zn(PPi)26-based negolyte at a high temperature?4. In flow batteries tests, why excess posolyte is used? 5. Why the battery using Zn2+ negolyte shown a much low coulombic efficiency (79%)?6.In line 194-202, Page 7, two parallel near neutral ZIFBs, 0.2 M ZnBr2 negolyte (pH=5.6), and the other with 0.2 M K6Zn(PPi)2 negolyte (pH=9.2),were employed.Is this pH difference affecting CE of the battery?The HER potentials of ZnBr2 negolyte and K6Zn(PPi)2 negolyte is suggested to provided.7.In line 157, Page 6, Zn2+ has a stronger interaction with PPi4-, how about the desolvation process of Zn(PPi)26-(or is the dissociation energy of Zn(PPi)26-on anode surface high?)?

Response to Reviewer#1
Overall Comment: Manuscript 463657 entitled "High Voltage and Dendrite Free Zinc Iodine Flow Battery" by Wang and colleagues reports on the development of a Zinc-pyrophosphate chelate and its use as negative electrolyte material in Zinc-iodine flow batteries.The approach is interesting and the paper is fairly well written, but I am not convinced by the claimed mechanism.The authors state that the chelate enables dendrite free plating of zinc.I suspect, however, that the zinc complex remains in solution and acts a "regular" dissolved redox active molecule and that the zinc plating that is observed is simply from uncomplexed zinc in solution.I can therefore not recommend publication of the article.If the authors can provide compelling evidence about the zinc plating behavior the manuscript could be reconsidered.
Response: Thank you very much for reviewing our manuscript and giving many constructive comments.We would like to answer your questions separately and revise the manuscript according to your suggestions.All the revisions according to your questions/suggestions are marked in blue in the revised manuscript.
For your major concerns given in the overall comment, we give a response as follows: (1) As correctly pointed out by you, the Zn plating process is based on the uncomplexed Zn 2+ in the electrolyte, which is accompanied by the dissociation of Zn(PPi)2 6-.In other words, the plating process consumes the free Zn 2+ , and simultaneously the dissociation of Zn(PPi)2 6-releases the free Zn 2+ .

Action:
We have revised the manuscript as follows: (Page 8~9 in the revised manuscript) "Note that the concentration of Zn 2+ is 1×10 -10 M in 0.1 M Zn(PPi)2 6-electrolyte based on the stability constant of Zn(PPi)2 6-.Correspondingly, φ Zn(PPi) 2 6− /Zn is calculated to be be -1.05V (vs.SHE) using the Nernst equation, which is very close to our experimental value." (3) The low concentration (~10 -10 M) of free Zn 2+ also alleviates undesired hydrogen evolution reaction (HER) during the Zn plating process, and consequently reduces the Zn dendrite growth.In conventional aqueous electrolytes containing large amounts of free Zn 2+ , the formation of hydrated Zn 2+ is inevitable.During the Zn-plating process, some H2O molecules in the solvated structure of Zn 2+ obtain electrons, resulting in HER.Such undesired HER not only reduces the Coulombic efficiency, but also leads to inhomogeneous Zn plating.In the subsequent Zn plating, the top effect of these inhomogeneous Zn deposits will aggravate the dendrite growth.In the Zn(PPi)2 6-electrolyte (e.g., 0.1 M ZnCl2 + 0.3 M K4PPi), the free Zn 2+ is kept at a very low concentration ( ~ 10 -10 M), and therefore H2O molecules remain within the free solvent network or coordinate with K + (1.2 M), rather than hydrated Zn 2+ (Recently, Chunsheng Wang's group has demonstrated a similar conclusion in their investigation about Zn plating/stripping behavior in a ZnCl4 2-based electrolyte, Nat.Sustain. 2023, 6, 325-335).As a result, the Zn(PPi)2 6-electrolyte can alleviate the undesired HER and facilitate smooth Zn plating.

Action:
We have revised the manuscript as follows: (Page 14 in the revised manuscript) "As a result, the Zn(PPi)2 6-electrolyte can alleviate the undesired HER and facilitate smooth Zn plating."

Main concern:
Question-1: L156: With such a strong binding energy, how is the chelate going to fall apart and plate zinc effectively?This is the first hint that you are not plating zinc but reducing the Zn-complex.
Response: Thanks for your question.The response is given as follows: (1) The Zn plating process depends on the free Zn 2+ (i.e., the uncomplexed Zn 2+ ), which is accompanied by the dissociation of Zn(PPi)2 6-.It means that the plating process consumes the free Zn 2+ , and simultaneously the dissociation of Zn(PPi)2 6-releases the Zn 2+ .In brief, the plating process does not involve the direct conversion between the chelated (Zn(PPi)2 6-) and metallic Zn.The concentration of free Zn 2+ in the electrolyte is controlled by the stability constant (K = 1×10 11 ) for Zn(PPi)2 6-.As shown in our response to your overall comment (or your question 2), the calculated concentration of free Zn 2+ is ~ 10 -10 M in the Zn(PPi)2 6-electrolytes.Based on the Nernst equation, we calculated the Zn-plating potential at the concentration (free [Zn 2+ ] ~1×10 -10 M), and obtained a redox potential of -1.05 V (vs.SHE), which is close to our experimental value of -1.08 V (vs.SHE) in the CV test (Fig. 3a).This result confirms that the Zn plating process depends on the free Zn 2+ (i.e., uncomplexed Zn 2+ ).
(2) In our opinion, the fast dissociation of the Zn 2+ -complex can provide enough free Zn 2+ to ensure the Zn 2+ consumption during the Zn plating process, which is confirmed not only by our experiments, but also by many previous reports.For example, in the widely reported and commercialized alkaline Ni-Zn batteries [i.e., Ni(OH)2/KOH electrolyte/Zn], the electrolyte contains a large amount of Zn(OH)4 2-and only trace amounts of free Zn 2+ .However, in such batteries, the fast Zn-plating (i.e., the high rate charge of Ni-Zn batteries) has been well demonstrated (J. Power Sources, 2001, 100, 125-148).Some recent work has also demonstrated the Zn-plating process in Zn 2+ -complex electrolytes, such as the ZnCl4 2-complex electrolyte (Nat. Sustain. 2023, 6, 325-335), and the EDTA-Zn(OH)3 -electrolyte (Energy Environ. Sci., 2024, 17, 717-726).

Action:
The related sentences are now provided in the revised manuscript as follows: (Page 8~9 in the revised manuscript) "Note that the concentration of Zn 2+ is 1×10 -10 M in 0.1 M Zn(PPi)2 6-electrolyte based on the stability constant of Zn(PPi)2 6-.Correspondingly, φ Zn(PPi) 2 6− /Zn is calculated to be be -1.05V (vs.SHE) using the Nernst equation, which is very close to our experimental value.This result indicates that the plating process of Zn(PPi)2 6-electrolyte consumes the free Zn 2+ , and simultaneously the dissociation of Zn(PPi)2 6-releases the free Zn 2+ ."during the zinc plating reaction the complex would have to fall apart forming free zinc which could then be plated.This may be somehow assisted by the pyrophosphate but it would still be Zn 2+ + 2e - → Zn 0 .
Response: Thanks for your valuable question.We quite agree with you that the Zn plating process depends on the free Zn 2+ (Zn 2+ + 2e -→Zn 0 ), and the low concentration of the free Zn 2+ in the Zn 2+ -complex electrolyte solution should be the key reason for the negative shift plating potential.As mentioned in the response to your overall comment, the concentration of free Zn 2+ in the electrolyte for the CV test is ~10 -10 M. According to the Nernst equation: the  ()  − / is calculated to be -1.05V, which is very close to our experimental value (-1.08 V vs. SHE).

Action:
We have revised the manuscript as follows: (Page 8~9 in the revised manuscript) "Note that the concentration of Zn 2+ is 1×10 -10 M in 0.1 M Zn(PPi)2 6-electrolyte based on the stability constant of Zn(PPi)2 6-.Correspondingly, φ Zn(PPi) 2 6− /Zn is calculated to be be -1.05V (vs.SHE) using the Nernst equation, which is very close to our experimental value.This result indicates that the plating process of Zn(PPi)2 6-electrolyte consumes the free Zn 2+ , and simultaneously the dissociation of Zn(PPi)2 6-releases the free Zn 2+ ."this does not at all look like a plating striping reaction.It looks much more like a "normal" CV of a redox active molecule in solution.

Question
Response: Thank you very much for your good equation.Yes, in the CV test with the conventional electrolyte containing a high concentration of free Zn 2+ , the cathodic current for Zn-plating (Zn 2+ + 2e -→Zn 0 ) continuously increases with negative sweep.This phenomenon is due to the fact that the free Zn 2+ can diffuse from the electrolyte bulk to the electrode surface at a very high rate, which efficiently compensates for the Zn 2+ consumption on the plating process.However, in the Zn 2+ -complex electrolyte, the Zn plating process involves two steps: 1.The Zn 2+ -complex diffusion from the bulk electrolyte to the near surface of the electrode, and 2. Dissociation of the Zn 2+ -complex to release free Zn 2+ for Zn plating.Generally, the diffusion rate of the Zn 2+ -complex is lower than that of the free Zn 2+ .When the diffusion of the Zn 2+ -complex cannot compensate the Zn 2+ consumption, the cathodic current reaches its maximum value to form a reduction peak (i.e., the normal CV as you mentioned).For example, the similar CV curves have also been demonstrated in the [ZnBr4] 2-complex electrolyte (Energy Storage Mater., 2022, 44, 433-440) and Zn(NH3)4 2+ complex electrolyte (J.Electrochem.Soc.2017, 164 (4), D230-D236).
Question-4: L258: Upon discharge any Zn 2+ released from the anode would immediately be chelated by the pyrophosphate?It seems as if this process should take some amount of time, as described in your synthesis section.
Response: Thank you for your question.Yes, the released Zn 2+ from the anode is indeed immediately chelated by the high concentration of PPi 4-due to the high stability constant of Zn(PPi)2 6-, which makes the free Zn 2+ at a very low concentration of ~10 -10 M. As you mentioned, we have ever described the electrolyte preparation as "ZnCl2 ….was added dropwise to the K4PPi solution".In fact, we slowly added 1 M ZnCl2 into the excess PPi 4-to prevent Zn2PPi precipitation which is caused by localized over-concentration of Zn 2+ .In the electrochemical process (i.e., the discharge process), Zn 2+ is released continuously in the high flowing electrolyte without the issue of localized over-concentration.
Question-5: Figure 5e: The intensity of plated Zinc is smaller at higher currents, this does not make any sense at all.Are you sure the "smooth" Zn you show in the SEM figure is not just uncomplexed Zn that is plated?
Response: Thank you for your good question.Herein we response your question as follows: (1) We believe that all the smooth Zn is derived from the free Zn 2+ (i.e., uncomplexed Zn) provided by the dissociation of Zn(PPi)2 6-.In other words, the plating consumes the free Zn 2+ , and simultaneously the dissociation of Zn(PPi)2 6-releases the free Zn 2+ .In the response to your overall comment, we have explained that the stability constant (K) for Zn(PPi)2 6-results in the presence of free Zn 2+ at a low concentration, which facilitate smooth Zn plating (Please see our response to your overall comment for detailed discussion).
(2) Fig. 5e illustrates the PXRD of deposited Zn at a constant current of 80 mA cm -2 , encompassing areal capacities ranging from 40 to 180 mAh cm -2 .Please note that we employ different carbon felts for separate Zn deposition, and therefore the relative intensities of the peaks may vary slightly.This variation could be associated with the differing degrees of compression in the carbon felts used each time.
Question-6: L322: Please explain why the 4-charged chelate is attracted more strongly to the negative electrode than a Zn 2+ cation or water?Please explain how the water activity is decreased?If there are less Zn 2+ ions that strongly bind water around themselves, how is the water activity lower ?
Response: Thank you for your good questions.Herein, we explain the ion adsorption on the pristine Zn surface (at the open circuit potential, OCP, without any charge), the ion adsorption on the Zn surface during the plating process (at the plating potential with electrons), and the water activity, respectively.See the follow points (1~3) for details.
(without any charge) prefers to adsorb PPi 4-, which is supported by the calculation (Figure 5f).As shown in Figure 5f, the calculated adsorption energies of PPi 4-and H2O molecules on the surface of Zn(101) are -1.05eV and -0.32 eV, respectively.It should be noted that the calculation is based on pristine Zn without any charge.In response to your question, we have compared the zeta potentials of zinc powder suspended in pure H2O, 0.8 M ZnBr2, and 0.8 M K4PPi in the revised supporting information (see the New Supplementary Fig. 27).As depicted in Supplementary Fig. 27, the zeta potentials of Zn are -0.45 mV (in pure H2O), 1.19 mV (in 0.8 M ZnBr2), and -6.20 mV (in 0.8 M K4PPi), respectively.The negative potential (-6.20 mV) strongly supports the calculation that pristine Zn prefers to adsorb PPi 4-.Such adsorption should be helpful for the formation of uniform nucleation on initial plating process.If the Zn surface adsorbs a lot of H2O molecules or hydrated Zn 2+ , the initial Zn-plating might involve electrochemical reduction of H2O (to generate H2), resulting inhomogeneous nucleation.

Action:
The related sentences have been added in revised the manuscript and supporting information as follows: (Page 14 in the revised manuscript) "In addition, the zeta potentials of zinc powders in H2O, ZnBr2 and K4PPi solutions were evaluated.
As depicted in Supplementary Fig. 27, the zeta potentials of Zn are -0.45 mV (in pure H2O), 1.19 mV (in 0.8 M ZnBr2), and -6.20 mV (in 0.8 M K4PPi), respectively.The negative potential (-6.20 mV) strongly supports the calculation result that pristine Zn prefers to adsorb PPi 4-."  is highly active at the interface.In contrast, in the Zn(PPi)2 6-electrolyte, the concentration of free Zn 2+ remains very low (approximately ~ 10 -10 M).Consequently, H2O molecules remain within the free solvent network or coordinate with K + , rather than forming hydrated Zn 2+ .Notably, Chunsheng Wang's group recently demonstrated a similar conclusion in their investigation of Zn plating/stripping behavior in a ZnCl4 2-based electrolyte (Nat.Sustain. 6, 325-335, 2023).

Action:
We have revised the manuscript as follows: (Page 14 in the revised manuscript) "As mentioned above, the plating process of Zn(PPi)2 6-electrolyte consumes the free Zn 2+ , and simultaneously the dissociation of Zn(PPi)2 6-releases the free Zn 2+ .Unlike the dendrite deposition mode of conventional Zn(H2O)6 2+ ions on the Zn surface (Fig. 5g) due to high interfacial water activity, 48 the interfacial water activity of Zn(PPi)2 6-ions is effectively reduced, allowing the subsequent dissociated Zn 2+ ions to plate on the Zn surface in an orderly manner in assistance with the PPi 4-ions (Fig. 5h)."Question-7: To convince me, please quantify how much Zn you plate from the Zn-chelate solution.
Response: Due to the high stability constant of Zn(PPi)2 6-, the mole fraction of  ( It means that all the Zn deposition arises from the Zn(PPi)2 6-anions.However, as we mentioned above, the Zn plating process is based on the uncomplexed Zn 2+ in the electrolyte, which is accompanied by the dissociation of Zn(PPi)2 6-.In other words, the plating consumes the free Zn 2+ , and simultaneously the dissociation of Zn(PPi)2 6-releases the free Zn 2+ .

Question
Action: According to you question, we have given a sentence to explain the advantage of ZFBs in the revised manuscript as follows: (Page 3 in the revised manuscript) "In addition to the fully soluble ARFBs mentioned above, zinc-based flow batteries have also made great strides in scaled energy storage due to the inexpensive zinc electrolyte, which can now reach the MW/MWh level. 12" The relative reference has been also added in the revised manuscript as follows:

Action:
We have revised the corresponding discussion in the revised manuscript as follows: (Page 3 in the revised manuscript) "As an illustration, all-vanadium ARFBs are currently the most widely commercialized RFB system. 6" The relative reference has been also added in the revised manuscript as follows: 6. Sun, C.; Zhang, H., Review of the development of first-generation redox flow batteries: Iron-chromium system.ChemSusChem, 2022, 15 (1), e202101798.

Question-11: L52: corrosion of what?
Response: Thank you for your question.In mild aqueous electrolytes with a pH value close to 7 (ranging from 3 to 11, for example), the presence of protons (H + ) may induce Zn corrosion through the chemical reaction: Zn + 2H + → Zn 2+ + H2.It appears that the term 'H2O-induced corrosion' may not accurately describe the process and has been replaced with 'proton-induced corrosion'.

Action:
We have revised the corresponding description in the revised manuscript as follows: (Page 3 in the revised manuscript) "Additionally, proton-induced corrosion, often characterized by hydrogen evolution, exacerbates the formation of 'dead' Zn, further diminishing the overall CE.Besides the challenges posed by Zn-dendrite growth and proton-induced corrosion, the crossover of Zn 2+ from the negolyte to the posolyte also limits the cycling stability of ZIFBs." Question-12: L55: Why is Zn-dendrite formation more pronounced at high currents?
Response: Thanks for your good question.Zn 2+ is deposited in the tip region as a result of an uneven electric field on the surface of the zinc anode.The higher the current density, the faster Zn 2+ is deposited, which leads to more severe concentration polarization and thus faster growth of zinc dendrites (ChemSusChem 2018, 11, 3996-4006).

Action:
We have mentioned this point in the revised manuscript as follows: (Page 3 in the revised manuscript) "It is widely recognized that the growth of Zn-dendrites on the anode becomes more severe at higher charging current densities (in mA cm -2 ), thereby elevating the risk of short circuits. 17" The relative reference has been also added in the revised manuscript as follows: 17. Lu, W.; Xie, C.; Zhang, H.; Li, X., Inhibition of zinc dendrite growth in zinc-based batteries.
Question-13: L57: Does the "dead Zn" dislodge and then flow through the cell?Does it clob the flowfields in any way?
Response: Thank you for your good questions.Yes, "dead Zn" can detach from the carbon felt and even clog the tube, leading to a rapid loss of capacity.We performed the cycling test of 0.2 M ZnBr2 based ZIFB, which couldn't work after several cycles.When we disassembled the cell, "dead Zn" clob the outlet hole, and plenty of loose "dead Zn" was found on the JCM-D membrane, as shown in  Question-14: L60: Can you explain the "H2O induced corrosion"?Why is Zn 2+ crossing into the positive side a problem, does Zn 2+ react with the cathode?
Response: Herein we answer your questions as follows: (1) In mild aqueous electrolytes with a pH value close to 7 (ranging from 3 to 11, for example), the presence of protons (H + ) may induce Zn corrosion through the chemical reaction: Zn + 2H + → Zn 2+ + H2↑.It appears that the term 'H2O-induced corrosion' may not accurately describe the process and has been replaced with 'proton-induced corrosion'.
(2) Zn 2+ ions shuttling to the catholyte do not react with the catholyte, but since Zn 2+ ions can't migrate back quickly enough to the negolyte, it will result in a decrease of the total Zn 2+ in the negolyte when zinc is deposited on the anode.

Action:
We have revised the corresponding description in the revised manuscript as follows: (Page 3 in the revised manuscript) "Additionally, proton-induced corrosion, often characterized by hydrogen evolution, exacerbates the formation of 'dead' Zn, further diminishing the overall CE.Besides the challenges posed by Zn-dendrite growth and proton-induced corrosion, the crossover of Zn 2+ from the negolyte to the posolyte also limits the cycling stability of ZIFBs."respectively.e The chelated process of Zn(PPi)2 6-ions.

Question-15:
(Page 6 in the revised manuscript) "Combining the above experimental results and subsequent theoretical simulations in Fig. 2, we depicted the formation process of Zn(PPi)2 6-in Fig. 1e.It is also found that the stability constant of Zn(PPi)2 6-is 1×10 11.0 . 27" The relative reference has been also added in the revised manuscript as follows: Question-20: L138: Why do you compare to ZnBr2?Because of Zn-Br flow batteries?I think it would be good to spend 1-2 sentences explaining this.
Response: Thanks for your good suggestion.Yes, we compare to ZnBr2 electrolyte because ZnBr2 electrolyte is usually utilized in zinc-based RFBs, and we also performed the parallel ZIFBs using ZnBr2 electrolyte and Zn(PPi)2 6-electrolyte in Figure 4a.

Action:
We have added a sentence in the revised manuscript as follows:

Action:
We have revised the manuscript as follows: (Page 8 in the revised manuscript) "In contrast, Zn(PPi)2 6-with negative ESP (Fig. 2h) is nucleophilic, which would contribute to the suppression of the by-product (e.g.Zn(OH)2 and ZnO) formation during zinc deposition." Question-24: L170: Can you elucidate why the LUMO of the complex is higher than the free Zn 2+ ?
Response: Thank you for your good question.Both Zn(PPi)2 6-and Zn(H2O)6 2+ are coordination complexes where zinc is the central metal ion, but they differ in their ligands.The basicity of the ligand can affect the corresponding LUMO energy level of the complex.For example, Laia Vilella et.increased successively with increasing ligand basicity.(Dalton Trans., 2011,40, 11241-11247).Here, PPi 4-ions are stronger bases than water molecules.This increased ligand basicity can lead to a stronger interaction with the positive zinc ion, affecting the electronic structure around the metal center.The stronger interaction can result in a higher LUMO energy for Zn(PPi)2 6-compared to Zn(H2O)6 2+ .

Action:
We have revised the corresponding description in the revised manuscript as follows: (Page 8 in the revised manuscript) "The results show that Zn(PPi)2 6-owns a higher LUMO energy (-0.17 eV) than Zn(H2O)6 2+ (-1.62 eV), which is attributed to the stronger ligand basicity of PPi 4-ions than water molecules. 35" The relative reference has been also added in the revised manuscript as follows: Question-25: L204: Why is it described as if surprising that Zn 2+ goes through a cation exchange membrane?What else would you expect?
Response: Thank you for your questions.We would like to emphasize that free Zn 2+ can permeate through a cation exchange membrane, whereas Zn(PPi)2 6-cannot.As previously mentioned (see response to question 14), Zn 2+ ions transferring to the catholyte do not undergo reactions in that environment.However, due to the limited speed of Zn 2+ ion migration back to the negolyte, there is a reduction in the total Zn 2+ concentration within the negolyte during rapid zinc deposition on the anode.This issue does not arise with Zn(PPi)2 6-negolyte.

Question-26:
Figure 4a: There are bumps in the blue curves at ca ., 20 mAh and 90 mAh.What is going on there?
Response: Thank you for your good question.In Figure 4a, we set the depth of charge of Zn(PPi)2 6- to be close to 100%, which means that the concentration of Zn(PPi)2 6-in the compact layer (see ) is very low after charging, and the coordination equilibrium at the surface of electrode would be broken.When the cell is discharged, the dissolution of Zn 2+ would recombine with the PPi 4-to construct a stable double electronic layer, which results in a decreased discharge voltage in the initial stage.For other rate or long cycle battery tests, we have set depths of charge close to 80%, there are no curve fluctuations observed.

Action:
We have added the relevant description in the revised manuscript as follows: (Page 11 in the revised manuscript) "The fluctuation in the GCD curves of Zn(PPi)2 6-based ZIFB may be due to the disruption of the coordination equilibrium of Zn(PPi)2 6-." Question-27: Figure 4f: Why is the charge polarization decreasing upon cycling?
Response: Thank you for your question.The charging polarization is reduced when zinc is deposited.
Since the Coulombic efficiency is slightly below 100%, the residual zinc will be in close contact with the carbon felt, the interfacial resistance of the electrodes is thus reduced, and accordingly the charging voltage plateau of the cell will be decreased after several cycles.

Action:
We have added the relevant description as follows: (Page 12 in the revised manuscript) "The charging voltage polarization decreases gradually after a few cycles due to the close contact of the residual zinc with the carbon felt." Question-28: L224: Again, why is it presented as surprising that the negatively charged chelate does not go through the cation exchange membrane?
Response: Thank you for your question.In theory, negatively charged complexes should be effectively isolated by cation exchange membranes.However, in practical applications, there may be trace amounts of negatively charged complexes that shuttle through the cation exchange membrane (Joule 2019, 3, 1-15).In our case, no detectable Zn(PPi)2 6-ions were observed after 30 days (Supplementary Fig. 14) in the reference cell, suggesting an exceptionally slow permeation rate of Zn(PPi)2 6-ions through the JCM-D membrane.
Question-29: L292: Coulombic interaction between the membrane and the electrolyte, why would this affect the dendrite formation?
Response: Thank you for your good question.Coulombic interaction between the membrane and the electrolyte can alleviate the Zn-dendrite formation, which has been reported by Li's group in 2018 (Ref. 47: Nat. Commun. 2018, 9, 3731) Response: Thank you for your good questions.This is the Zn nucleation overpotential (We marked it in the revised supporting information, labeled as Supplementary Fig. 25) at the initial stage, but not the real overpotential for the overall charging process.To show this more clearly, we have modified the horizontal coordinate in Figure 4a to start form -1 mAh.The charging polarization for Zn(PPi)2 6- based ZIFB is still high up to 50 mV at the initial stage, which is larger than 30 mV of the Zn 2+ based ZIFB, but it decreases rapidly once Zn is deposited.The whole overpotential of the two cell is close to each other, which can be seen from their voltage difference between charging and discharging plateaus in Fig. 4a.

Action:
We have revised the supporting information as follows: Response: Thank you for your good suggestion.

Action:
We have supplemented a sentence in the revised the manuscript as follows: (Page 15 in the revised manuscript) "Further efforts should focus on increasing the cell voltage (by utilizing the high-potential electrolytes) or improving the cycling stability under ultra-high deposited zinc areal capacity to achieve further breakthroughs in energy density or long duration energy storage."

Response to Reviewer#2
Overall Comment: This manuscript presented a very nice work on negolyte development for zinc-iodine flow battery.Author's innovative refreshing chemistry by chelating K4P2O7 with Zn 2+ has produced a negolyte that enabled a high-voltage and dendrite-free zinc-iodine flow battery, performing significantly better than conventional zinc-iodine flow battery in terms of working current density and areal capacity.Their effort and novelty are to be commended, which will have important impact for flow battery technology.I recommend the manuscript to be accepted and published in Nature Communications.
Response: Thank you very much for recognizing our manuscript.We would like to answer your questions separately and revise the manuscript according to your suggestions.All the revisions according to your questions/suggestions are marked in purple in the revised manuscript.
Response: Thank you for your good question.The solubility of Zn(PPi)2 6-electrolyte prepared from ZnBr2 is 0.7 M at room temperature with a [PPi 4-]:[Zn 2+ ] ratio of 3:1, which is lower than that of Zn(PPi)2 6-prepared from ZnCl2.
Action: According to your question, we have added the relevant sentence in the revised manuscript as follows: (Page 6 in the revised manuscript) "Besides, when ZnBr2 precursor is used, the solubility of the prepared Zn(PPi)2 6-solution will decrease to 0.7 M with a [PPi 4-]:[Zn 2+ ] ratio of 3:1." Question-2: For the permeability measurement of Zn 2+ , the right compartment was filled with 20 mL of 0.4 M KCl, while in Supplementary Fig. 11's caption, saturated zincon monosodium salt is added in the reference cell, is the zincon monosodium salt added into the right compartment prior to the permeability measurement?
Response: Thank you for your good question.Yes, the saturated zincon monosodium salt reagent is added to the right-hand cell in advance, and if trace amounts of zinc ions diffuse from the left compartment, the zincon monosodium salt (yellow in colour) will coordinate with the Zn 2+ and the solution will appear in red colour.

Action:
We have added related information in the revised manuscript as follows: (Page 17 in the revised manuscript) "For the permeability measurement of Zn 2+ , the left compartment of the diffusion cell was filled with 20 mL of 0.2 M ZnBr2, while the right compartment was filled with 20 mL of 0.4 M KCl, and saturated zincon monosodium salt reagent was added to right compartment in advance." Question-3: How about the stability of Zn(PPi)2 6-based negolyte at a high temperature?
Response: Thanks for your good question.We prepared the saturated Zn(PPi)2 6-solution by rotary evaporator at 50 °C, and the high-concentration negolyte can be stably operated.Besides, we heated 0.2 M Zn(PPi)2 6-negolyte at 80 °C over 48 h, no precipitation was found.

Action:
We revised the related information in the revised manuscript as follows: (Page 15 in the revised manuscript) "The resulting chelated Zn(PPi)2 6-solution was stirred continuously until the solution became transparent, and then concentrated to 45 mL under reduced pressure at 50 °C." Question-4: In flow batteries tests, why excess posolyte is used?
Response: Thanks for your good question.The KI posolyte may generate solid I2 during cycling at high state of charge (SOC), leading to its deposition on the carbon felt and potential pipe clogging.
However, the excess KI can coordinate with the I2, preventing the formation of I2-deposits on the carbon felt.In addition, KI itself is slowly oxidized by air and therefore usually needs to be stored away from light.For these two reasons, we add excess posolyte.
Response: Thanks for your good question.As shown in Figure 5a, the flow battery using Zn 2+ negolyte exhibits a low Coulombic efficiency of 79%.This phenomenon arises from two primary reasons.Firstly, some Zn 2+ ions diffuse from the negolyte to the posolyte, crossing the cation-ion membrane during the charging process.Secondly, the electrochemical hydrogen evolution reaction (HER) occurs in the negolyte during the charging process.Please refer to our response to question-6 for detailed information.Response: Thank you for your good suggestion.In our opinion, the low CE of ZnBr2 based ZIFB should arise from the crossover of Zn 2+ from the negolyte to the posolyte and the electrochemical hydrogen evolution reaction (HER) occurs in the negolyte during the charging process.In Figure 4a, these ZIFBs were charged with a fixed capacity of 100 mAh, calculated based on the theoretical capacity of the Zn 2+ ions in the negolyte.For the ZnBr2 based ZIFB, before the cell was charged at 100% SOC (based on the used ZnBr2), partial of Zn 2+ ions had been permeated from the negolyte to the catholyte, and therefore H + will be reduced (i.e., HER) after the deposition of Zn 2+ ions.
According to your suggestion, we also performed the LSV test of the 0.2 M ZnBr2 and 0.2 M Zn(PPi)2 6-negolyte on 1 cm -2 carbon paper at a scan rate of 5 mV s -1 .The results showed that the HER in the 0.2 M ZnBr2 negolyte was more severe than that in 0.2 M Zn(PPi)2 6-.

Action:
We have revised the manuscript as follows: (Page 11 in the revised manuscript) "Linear sweep voltammetry (LSV) of the two negolytes was then performed, and the result showed that the hydrogen evolution reaction (HER) in the 0.2 M ZnBr2 negolyte was more severe than that in the 0.2 M Zn(PPi)2 6-negolyte (Supplementary Fig. 8)." (Page 9 in the revised supporting information) Supplementary Fig. 8 LSV profiles of the 0.2 M ZnBr2 and 0.2 M Zn(PPi)2 6-negolytes on carbon paper (1 cm -2 ) at a scan rate of 5 mV s -1 .
Response: Thank you for your good question.The dissociation process of Zn(PPi)2 6-means that it decomposes to Zn 2+ and PPi 4-.We found that the stability constant of Zn(PPi)2 6-is 1×10 11.0 , and this value is much lower than that of Zn(OH)4 2-(1×10 17.6 ).Note that the latter could plate on the carbon felt in a rapid pace.Hence, it is anticipated that the desolvation process of Zn(PPi)2 6-is not too high.
The authors have done a very good job revising the manuscript and answered many of my questions in their response, for example the zeta potential part is a great addition.The manuscript contains a lot of hard work, suitable experiments and the authors have well explained the low zinc potential induced by very low free-Zn concentration, which in turn is caused by the high stability of the complex.However, some key points are still unclear to me and I require further explanations before I can recommend publication of the proposed mechanism: The complex has a stability constant of 1011, it is quite stable indeed.Accordingly, there is only 10-10 free Zn2+.You suggest that this is the main reason why the system works without dendrites and at the observed low potential.Since only free Zn2+ is plated, the complex has to dissociate rapidly to provide fresh Zn2+ to be plated.But what is the driving force for this dissociation?If you charge at high currents, a lot of complex has to dissociate very rapidly to provide enough plate-able Zn2+, but with such a stability constant this seems unlikely.Why is enough complex falling apart to provide enough Zn2+ at high rates?If it is presumed to be simply equilibrium driven (ZnPPi2 <-> Zn2+ + 2PPi, if free Zn2+ is consumed and plated the equilibrium shifts to the right side so some of the complex dissociates to provide fresh Zn2+) a much less stable complex would be required to deliver enough Zn2+ in time to be plated at 200 mA/cm2.What makes the complex fall apart so easily even though it has a stability constant of 1011?Can you calculate how many moles of zinc you are plating per second at 200 mA/cm2?This would inform us about how much of the complex has to dissociate in said amount of time.Essentially, this is what you see in cyclic voltammetry, not enough Zn2+ is provided at some point resulting in a cathodic current maximum.
During discharge, on the other hand, you state that any Zn2+ released from the anode is immediately complexed.At high currents, there is locally a very large amount of Zn2+ stripped from the electrode, which according to the description of your synthesis should form insoluble Zn2PPi.You claim that there is no local over-concentration of Zn2+ upon discharge and that all Zn2+ is immediately consumed to form ZnPPi2 but as you describe the synthesis of ZnPPi2 there is a certain time component.Can you calculate, again, how much zinc is stripped from the anode per second at 200 mA/cm2?Of course, convection in the flow cell helps here.Can you do zinc plating in a static three electrode setup?I would expect the maximum plating rate to be significantly lower.
The high binding energy of PPi to Zn seems to make it a great additive to modulate Zn plating/striping.However, the proposed mechanism, i.e., complex dissociation at a rate proportional to free Zn2+ consumption and vice versa complex formation as soon as Zn2+ is stripped does not convince me.Can you please explain if I am misunderstanding something?Additional comments: -I think you should include your explanations to my major concerns 2 and 3 (Nernst and suppressed HER) in the SI -You should state in the text that your calculations (Figure 5f) are based on Zn without any charge -In Figure 1d you discuss vs(PO3) and vas(PO3) which obviously shift a lot upon complex formation.Is it possible to isolate the P=O vibrations in IR/Raman and compare those to the bond lengths you calculate?-Answer to question 19: I think this should be included in the SI -Figure S4: You state that experimental and calculated spectra fit very well, which I think is a bit of a stretch in this case.Where are the discrepancies coming from?-Figure S6 is missing -The zincon discussion in your response to reviewer 2 could be included in the SI -English needs work Reviewer #2 (Remarks to the Author):

(
Page 16 in the revised manuscript) "Zeta potential measurements were performed by Nanometrics (ZEN3690)."(Page 28 in the revised supporting information) Supplementary Fig. 27 Zeta potential of zinc powder in various solutions.(2) During the plating process, the Zn electrode gains electrons (i.e., becomes negatively charged), and the ion absorption is illustrated in Figure Answer 1.As depicted in Figure Answer 1, the surface adsorption on the Zn electrode (with negative charges) includes the inner layer of free Zn 2+ and the outer layer of ZnPPi2 6-.The plating process consumes the free Zn 2+ in the inner layer, while simultaneously, the outer layer of ZnPPi2 6-provides the necessary free Zn 2+ through a dissociation reaction.

Figure Answer 1 .
Figure Answer 1. Schematic illustration of the ions adsorption on the anode during the plating process.(3)In conventional aqueous electrolytes containing Zn 2+ , the Zn 2+ ions are surrounded by H2O molecules, forming hydrated Zn 2+ complexes, such as [Zn(H2O)6] 2+ .During the Zn-plating process, some H2O molecules in the solvation structure of Zn 2+ gain electrons, leading to the occurrence of the hydrogen evolution reaction (HER).This indicates that the H2O in the solvation structure of Zn 2+

- 8 :
L40: the separation of energy and power components is largely lost with a plating/striping electrode like zinc Response: We acknowledge that the adjusting ability of Zinc-based flow batteries (ZFBs) for energy and power separation is lower compared to fully soluble flow batteries, such as All-vanadium flow batteries.However, the operational flexibility of ZFBs still surpasses that of conventional rechargeable batteries in adjusting power and energy.Notably, the environmentally friendly and low-cost Zinc-based electrolyte makes ZFBs an attractive option.As a result, ZFBs are garnering extensive attention and have successfully demonstrated applications at the MW/MWh level (Mater. 12. Khor, A.; Leung, P.; Mohamed, M. R.; Flox, C.; Xu, Q.; An, L.; Wills, R. G. A.; Morante, J. R.; Shah, A. A., Review of zinc-based hybrid flow batteries: From fundamentals to applications.Mater.Today Energy 2018, 8, 80-108.Question-9: L42: what structural features?Response: Thanks for your question.ARFBs features excellent scalability, modular manufacturing and flexible design.Action: We have revised the related sentence in the revised manuscript as follows: (Page 3 in the revised manuscript) "Moreover, ARFBs can decouple power and energy, all while meeting stringent safety requirements due to the features of excellent scalability, modular manufacturing, flexible design, as well as the non-flammability of aqueous electrolytes. 5" Question-10: L44: VRFB have developed far beyond demonstration purposes with 100's of MWh installed globally Response: Yes, VRFBs are in the commercialization stage.

Figure
Figure Answer 2.

Figure
Figure Answer 2. a, Photograph of "dead Zn" blocking the outlet hole.b, Photograph of "dead Zn" on the JCM-D membrane.
Page 7 in the revised manuscript) "Here, we chose ZnBr2 electrolyte for comparison because it is widely used in zinc-based flow batteries."Question-21: L148: Is this concentration dependent?Response: Thanks for your question.The chemical shifts of substances at different concentrations could not change.See Magn.Reson.Chem.2018; 56: 1124-1130.Question-22: L160: Please explain ESP with 1-2 sentences.Response: Thanks for your good suggestion.ESP (Electrostatic Potential) of a molecule refers to a physical quantity that describes the properties of the electrostatic field surrounding the molecule, which is usually calculated to understand the molecular reactivity.Action: We have revised the relevant sentences in the revised manuscript as follows: (Page 8 in the revised manuscript) "To understand the charge distribution and electron density of two species, electrostatic potential (ESP) mapped molecular van der Waals surfaces of them were also calculated.The results exhibit total different electric inherent, i.e., positive ESP for Zn(H2O)6 2+ ion, while negative ESP for Zn(PPi)2 6-ion."Question-23: L165: The molecule has a 6-charge but it is electrophilic?How does that work?Why do you reference Fig 2d here?Response: Thank you for pointing out our wrong description.The Zn(PPi)2 6-with negative ESP should be nucleophilic, rather than "electrophilic".In our negolyte, Zn(PPi)2 6-(with negative ESP) seldom combines with OH -to form Zn(OH)2 precipitates.In contrast, Zn(H2O)6 2+ (with positive ESP) in conventional electrolytes can react with OH -to generate undesired Zn(OH)2 precipitates.Besides, we mistakenly referenced Figure2dhere.In fact, Figure2hshould be referenced here to indicate the negative ESP around Zn(PPi)2 6-ion.
. Li et al. have demonstrated that Zn(OH)4 2-negolyte easily formed Zn dendrites upon deposition using non-charged porous PES membrane, while it didn't grow dendrites using porous PES/SPEEK composite membrane with negatively charged groups.The