A safe and non-flammable sodium metal battery based on an ionic liquid electrolyte

Rechargeable sodium metal batteries with high energy density could be important to a wide range of energy applications in modern society. The pursuit of higher energy density should ideally come with high safety, a goal difficult for electrolytes based on organic solvents. Here we report a chloroaluminate ionic liquid electrolyte comprised of aluminium chloride/1-methyl-3-ethylimidazolium chloride/sodium chloride ionic liquid spiked with two important additives, ethylaluminum dichloride and 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide. This leads to the first chloroaluminate based ionic liquid electrolyte for rechargeable sodium metal battery. The obtained batteries reached voltages up to ~ 4 V, high Coulombic efficiency up to 99.9%, and high energy and power density of ~ 420 Wh kg−1 and ~ 1766 W kg−1, respectively. The batteries retained over 90% of the original capacity after 700 cycles, suggesting an effective approach to sodium metal batteries with high energy/high power density, long cycle life and high safety.

propyl-N-methylpyrrolidinium bis(fluorosufonyl)imide, or C3mpyrFSI. This type of comparison would be more insightful than showing results for two similar positive electrodes (NVP@rGO and NVPF@rGO).

Specific comments
For secondary (rechargeable) batteries, it is recommended to use the terms 'negative' and 'positive' for the cell electrodes, instead of 'anode' and 'cathode.' The procedure for preparing graphene oxide is mentioned as a modified Hummer's method (line 373). What are the details of the method actually employed? Different cell assemblies were used, with different materials for the positive electrode. How was Na foil used as positive electrode (line 398)?
Supplementary figure 14 was described as TGA profile for NVPF@rGO in the text (line 221), but subtitled as TGA profile for NVP@rGO. In any case, it is unclear how the rGO percentage was determined in the sample, since there is continuous weight loss until ~450 oC. It would be interesting to specify the temperature range used.

Remarks to the Author: This manuscript entitled "A Safe, Non-Flammable Sodium
Metal Battery Based on a Novel Ionic Liquid Electrolyte" presented a novel electrolyte based on chloroaluminate ionic liquid for safe sodium metal battery. The proposed ionic liquid electrolyte was fire-resistant and had a high ionic conductivity. Furthermore, the additives were demonstrated to be crucial for the reversible sodium plating and stability of NVPF hybrid electrode. The concept is new, and the results are satisfying. Therefore, I would like to recommend it to be published after some minor revisions and addressing some issues below.

Response:
We thank the reviewer's positive/insightful comments. The comments and suggestions have helped a lot to further strengthen this work. The manuscript has been carefully revised according to your valuable suggestions.
(1) The chloroaluminate ionic liquids have high tendency to adsorb water. What method does the author use to keep them water-free, besides heating to dry? If possible, a water content report is preferred for this matter.
Response: Thank you very much for your comments and advices. Water content report was not provided because when injecting the chloroaluminate ionic liquids into a Karl Fischer titrater, precipitation was observed in the testing electrolyte containing iodine-based species, probably due to the reaction with AlCl 3 -based species. Actually, any water in the Na-Cl-IL will react with aluminum chloride-based species and become HCl, which was supposed to be removed by heating under vacuum and reacting with EtAlCl 2 .

Revisions made:
We added more details to ensure the ionic liquid electrolyte water-free which was described at the last paragraph of Page 11, as follows: "To avoid water absorption of the prepared IL electrolyte, all the agents were stored inside tightly closed and sealed bottles, and placed in Ar-filled glove box.
[EMIm]Cl and NaCl were dried via heating under vacuum before use." (2) One of the problems of using ionic liquid is that the effective transference number of Na+ may not be too high, even though they possess high ionic conductivity, especially Al-based salt is also present. The author should give a relative detection of the sodium ion transference number.
Response: Thank you very much for the insightful suggestion. The Bruce-Vincent method that is generally used might need some modification to accurately reveal the sodium ion transference number of the Na-Cl-IL electrolyte, particularly with the presence of Al-based salt as you mentioned, which makes the system more complicated than conventional organic and IL electrolyte systems. As a result, we have not included the sodium ion transference number herein, but will design more experiments to investigate this system in the future. Thanks again for your comments and advices!

(3) It is understandable that the [EMIm]FSI has been demonstrated to be critical role of Na plating/stripping, and the sole [EMIm]FSI is not suitable for Na/NVP@rGO cell. Is it possible using [EMIm]FSI as solvent and EtAlCl2 as additive for the cell?
Response: We performed additional experiments to address this issue. Using [EMIm]FSI as solvent (e.g., 50 wt% in the Na-Cl-IL electrolyte) made the obtained electrolyte highly viscous, and the Na/NVPF@rGO battery showed large overpotential and low specific capacity as below. Figure R1. Galvanostatic charge-discharge curves of a Na/NVP@rGO cell using Na-Cl-IL electrolyte with 50 wt% [EMIm]FSI.
In addition, we also made a Na/NVP@rGO cell using 1 M NaFSI in [EMIm]FSI (as solvent) IL electrolyte. It showed low and fluctuating CEs as below, indicating less stable SEI formed for reversible Na plating and stripping than our buffered IL electrolyte. Figure R2. Capacity and Colombic efficiency retention of a Na/NVP@rGO cell using 1 M NaFSI in [EMIm]FSI IL electrolyte at 150 mA/g. Please note that another factor/consideration is that due to the high price of [EMIm]FSI, it is preferable to reduce its amount in forming electrolytes to achieve high performance and low cost at the same time.

Remarks to the Author: Most of the work on sodium-ion batteries involves the use of organic solutions as electrolytes, in spite of their safety problems. It is important to find suitable safe electrolytes for these devices. In this regard, the work shows interesting results and a thorough investigation of the Na-Cl-IL electrolyte when used in a Na-ion battery, specially the composition of the SEI layer.
Response: We thank the reviewer's positive/insightful comments. By addressing the issues raised, we have strengthened this work substantially. We have also investigated the cyclic stability and rate performances of Na/NVP@rGO cells using NaFSI/N-propyl-N-methylpyrrolidinium bis(fluorosufonyl)imide IL electrolyte as a comparison according to your valuable suggestions.
(1) AlCl4-[EMIm]+ ionic liquid has been studied for a long time, and its propertiessuch as high stability, non-flammability and high conductivity -are well established. Ionic liquids high stability in comparison to organic solvents, in particular, is the main reason why ILs have been studied as Li-ion and Na-ion battery electrolytes. The good results found for the NVPF@rGO|Na cell are not necessarily due to the high conductivity of the ionic liquid used as electrolyte. It is desirable to make a comparison among different ionic liquids using the same cell assembly and the same positive electrode material. One suggestion is the N-propyl-N-methylpyrrolidinium bis(fluorosufonyl)imide, or C3mpyrFSI. This type of comparison would be more insightful than showing results for two similar positive electrodes (NVP@rGO and NVPF@rGO).
Response: Thank you very much for your constructive suggestions. We performed additional experiments to address this issue. Na/NVP@rGO cells were made using NaFSI/N-propyl-N-methylpyrrolidinium bis(fluorosufonyl)imide (molar ratio: 2:8) IL electrolyte for comparison of both rate capability and cyclic stability.