Rapid mechanochemical synthesis of polyanionic cathode with improved electrochemical performance for Na-ion batteries

Na-ion batteries have been considered promising candidates for stationary energy storage. However, their wide application is hindered by issues such as high cost and insufficient electrochemical performance, particularly for cathode materials. Here, we report a solvent-free mechanochemical protocol for the in-situ fabrication of sodium vanadium fluorophosphates. Benefiting from the nano-crystallization features and extra Na-storage sites achieved in the synthesis process, the as-prepared carbon-coated Na3(VOPO4)2F nanocomposite exhibits capacity of 142 mAh g−1 at 0.1C, higher than its theoretical capacity (130 mAh g−1). Moreover, a scaled synthesis with 2 kg of product was conducted and 26650-prototype cells were demonstrated to proof the electrochemical performance. We expect our findings to mark an important step in the industrial application of sodium vanadium fluorophosphates for Na-ion batteries.

7. The English is very poor. For example, " Fig. S3. The results show that the best ratio of V: P: F is 1: 1.5: 1 owing to the best crystallinity and a high yield of 94%." The whole manuscript should be carefully revised by a native English speaker.
Reviewer #4 (Remarks to the Author): This work of Shen and co-workers investigates the synthesis of polyanionic cathode with a good performance for Na ion batteries The authors present a very detailed and well-researched study on an important class of materials. There is also an usually large variety of complementary techniques used, and each method is difficult to perform. In terms of energy density, obtaining such a amount of W h kg-1 is the key point for future development of na ion batteries. However, for industrial point of view more test should be necessary to afford its commercialization (then the energy density will be lowered). Through characterization data are provided and the results are well aligned with previous reports in literature. This cathode has two principal voltage reactions around 3.5 and 4.1 V. There are many reports in literature dealing with sodium vanadium phosphates and sodium vanadium fluorophosphates which exhibit the same performance. In my opinion the authors don't provide new data to be considered as of high impact to be published. Also, the full cell performance is very similar to that with compounds hard carbon / 1M NaClO4 in PC+(FEC) / Na3V2(PO4)2F3. Overall, this is a well-designed study and a well-written manuscript suitable for publication in a highimpact journal, but does not reach the level required for Nat. Commun. in terms of novelty or groundbreaking conclusions or wide-reaching impacts.
Thank you for the reviewers' comments concerning our manuscript (NCOMMS-20-26882). Those comments are extremely valuable and helpful for revising and improving our paper, as well as the important guiding significance to our research. We have studied the comments carefully and have made corresponding corrections. We have revised the MS as suggested. The changes made in the manuscript have been highlighted in red color in the revised version and all the points raised by the reviewers have been addressed point by point. Please check the revised MS as well as our reply to the referees in the following page.
Our itemized responses to the reviewers' questions, comments and suggestions are as follows: Reviewers' comments:

Reviewer #1 (Remarks to the Author):
This work successfully synthesized NVPF compound and its carbon composite by a mechanochemical method using high energy ball milling. The developed NVPF/KB composite exhibits excellent electrochemical performance, including high specific capacity, rate capability, and cycling stability. In addition, they constructed a full cell of NVPF/KB cathode and hard carbon anode with Na 2 C 4 O 4 additive. The full cell shows a good cycling stability. While the electrochemical performance of the developed NVPF/KB cathode is excellent, this reviewer cannot find a significant novelty in their approach. The mechanochemical method has been used for the synthesis of various electrode materials for rechargeable batteries. In addition, it is hard to agree with that mechanochemical method using high energy ball milling is scalable to industrial levels. In fact, many battery industry people are very skeptical to use high energy ball milling for commercialization. In this respect, this reviewer feels that this manuscript is not suitable for a high standard journal like Nature

Communications.
Reply: Thank you for your comments. As we know, for many electrode materials, mechanochemical method has been widely used as an assistant procedure in the synthetic process, which is only for precursors, not for target products, such as the precursors for LiFePO 4  prototype cells with hard carbon as anode, as shown in the revised Figure 5.
Here are minor points: (i) It is really hard to read scale in Figure 2c-d.

Reply: Thanks for your kind reminder.
We have carefully checked the scale marker for Figure 2c-d and redrew them in the revised manuscript.
(ii) Can the authors explain why the carbon composite delivers higher capacity than theoretical value? Does the high energy ball milling with carbon introduce defects in NVPF for extra capacity? Given extra capacity comes from low voltage ( Figure S8a), the extra capacity might result from defects or amorphized structure/material.

Reply: Thanks for your question.
As seen in Fig. S9 with superior electrochemical performance. The material characterization seems to be enough to support their data. My overall view of the paper is that while the work is of potential interest for publication after the following revision.
Reply: Thank you so much for your positive comment.
The detailed comments are listed below.
1. The mechanism of mechanochemical synthesis should be presented in detail.
During the mechanochemical synthesis, what are the parameters followed? Why the mechanochemical synthetic method could produce highly crystalline and pure NVPFs?
All the impurities or unreacted precursor are soluble in water? How did the impurities and unreacted precursor were removed?
Reply: Thanks for your good comments.
The mechanism of mechanochemical synthesis does require more details. The specific raw material ratios were listed in Table S1. For better understanding, we have added the detailed synthetic parameters of NaVO 3 as vanadium source in the section of 'Methods' in the manuscript.
Mechanochemical method has been developed as a high-energy synthetic process in recent years 5,6 . Compared with traditional synthesis method, it produces new materials by using mechanical energy to induce chemical reactions or structure changes. Mechanochemical strategy could significantly reduce the reaction activation energy, refine the grain, and enhance the combination of the interface and substrate to achieve a low-temperature chemical reaction 7,8 . In fact, the mechanochemical reaction is equivalent to a kind of high-salt medium reaction, and it is much simpler than a co-precipitation method concerning reaction time and space.

Reviewer #4 (Remarks to the Author):
This work of Shen and co-workers investigates the synthesis of polyanionic cathode with a good performance for Na ion batteries Overall, this is a well-designed study and a well-written manuscript suitable for publication in a high-impact journal, but does not reach the level required for Nat.
Commun. in terms of novelty or groundbreaking conclusions or wide-reaching impacts.  Figure S12c and Table S8. To the best of our knowledge, the tested electrochemical performances of our sample is the best, and the HC// Na 3

V 2 (PO 4 ) 2 F 3 /KB prototype cells with an energy density of 88 Wh kg -1 were exhibited, which is superior to the reported HC// Na 3 V 2 (PO 4 ) 2 F 3 full cell (75
Wh kg -1 ) 14 . The optimization of full-cell performance still has a way to go by coupling advanced anode materials and an even better electrolyte. For the scale up, it is impressive that the authors show the production of their material in 2-kg scale. However, as the authors might know, the practical synthesis of electrode materials is not in a few kgs-scale. While their demonstration is interesting and insightful, it does not indicate that their method is practical for mass (tons) production. But, the revision of this part and demonstration of kgs-scale synthesis is great.

References
Lastly, their argument on the extra-capacity from interface between NVPF/KB is really over-stated. They did not provide any direct evidences if the extra capacity is really coming from the interface. Of course, it is possible, but no evidence is provided.
Reviewer #2 (Remarks to the Author): All the reviewer's concern is well responded. Reviewers' comments:

Reviewer #1 (Remarks to the Author):
The authors claimed that mechano-chemical method has been used as an assistant procedure in the synthetic process, but not for target materials. However, this is not true.
In fact, there are many examples that use "real" mechano-chemical reaction (without heat treatment) to synthesize electrode materials for Li and even Na-ion batteries. For the scale up, it is impressive that the authors show the production of their material in 2-kg scale. However, as the authors might know, the practical synthesis of electrode materials is not in a few kgs-scale. While their demonstration is interesting and insightful, it does not indicate that their method is practical for mass (tons) production.
But, the revision of this part and demonstration of kgs-scale synthesis is great.
Lastly, their argument on the extra-capacity from interface between NVPF/KB is really over-stated. They did not provide any direct evidences if the extra capacity is really coming from the interface. Of course, it is possible, but no evidence is provided.  (Supplementary Fig. 9a). Then, we investigated the Nastorage capacity for the discharged KB with and without ball-milling. It can be found that the discharged capacity of KB with and without ball-milling are 3.5 and 0.7 mAh g -1 , respectively, which proves the Na-storage of ball-milled KB itself ( Supplementary   Fig. 9b-c). The X-ray photoelectron spectroscopy was employed to detect the chemical state of discharged KB electrodes, as shown in Supplementary Fig. 9d. No Na signal was observed in the discharged KB without ball-milling, but an obvious Na 1s peak located at ~1069.2 eV can be found in the discharged KB with ball-milling, which confirms the binding effect between Na and ball-milled KB. Based on this, we collected the Na 1s XPS data of NVOPF and NVOPF/8%KB electrodes with varied charge/discharge states. As shown in Supplementary Fig. 10a 14,15 Inspired by the extra Li-storage, the current interfacial Na-storage can be illustrated in Supplementary Fig.   11. In the interface between Na3(VOPO4)2F and KB, Na + can be accommodated at the boundary of NVOPF side while the electrons are restricted to the KB side. For the combination of Na3(VOPO4)2F and KB, the stored Na + and eact as a bridge during the charge-discharge process. 16 In this case, an interfacial Na-storage effect can be