After investigations on many kinds of fluorides, carbon fluoride (or graphite fluoride, CF_x) is the best material as cathode of primary lithium battery with ultrahigh energy density ^[1-2], while F₂ gas is difficult to operate as cathode. And many attempts for extending the application concepts of CF_x in lithium batteries were made to conclude no any evidence on its reversibility^[3]. Since sodium batteries have been attracting much attention as one of the post-lithium ion battery^[4]. CF_x/Na cells were explored and the reversibility of these cells was also proved^[5-6].

Here, we summarize these designed CF_x materials for sodium secondary batteries, as shown in Figure 1. Commercial CF_x and F-MWCNTs were synthesized by direct fluorination of graphite and mutil-wall carbon nanotubes (MWCNTs) using F₂ gas. While CGF powders were prepared by simply ball-milling of the commercial CF_x, FeF₃ and GNS, with higher surface area, electronic conductivity, more active sites and specific channels for the transportation and storage of fluorine ions. Furthermore, disordered CF_x exhibits excellent electrochemical performance due to its intrinsic amorphous character.

Figure 1 Three kinds of CF_x materials for sodium secondary batteries:(a) commercialCF_x, (b) CF_x/GNS/FeF₃(CGF) composite, (c) F-MWCNTs

Commercial CF_x was first used in sodium secondary battery^[5]; however, much larger overpotential(2000mV) between charge and discharge voltage and terrible cyclic performance (5 cycles) hindered its practice adoption. These drawbacks can be overcome by the introduce of catalysts and the change of the intrinsic characteristic of CF_x materials^[5-6], which results in a gap of only 0.8 volt and enhanced cycle life.

For practical uses, some popular approaches in nanotechnology and electrolyte chemistry such as the core-shell structure or encapsulation or coating composites, and using specially designed solid electrolytes will be expected to prevent the dissolution of fluorine for improving cycle life for Na-CF_xbattery in the future.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (NSFC21373137) and Shanghai Science and Technology Development Fund (15DZ2282000).

Reference

[1] N. Watanabe, M. Fukuda, U.S. Patent 3 536 532, 1970 and 3 700502, 1972.

[2]  M. Fukuda, Iijima, T. In Collins, D.H., Ed.; Power Sources 5;Academic Press: New York, 1975; p 713.

[3] G. G. Amatucci, N. J.  Pereira, Fluorine Chem. 2007, 128, 243.

[4] D. S. Michael, K. Donghan,L.  Eungje, and S. J. Christopher, Sodium-Ion Batteries, Adv. Funct. Mater. 2013, 23, 947.

[5]   W. Liu, H. Li, J. Y.Xie, and Z. -W. Fu, ACS Appl. Mater. Interfaces, 2014, 6, 2209.

[6]   W. Liu, Z. Shadike, Z. C. Liu, W. Y. Liu, J. Y.Xieand Z. -W. Fu, Carbon, 2015, 523.

[7]   Y. Shao, H. Yue, Z. Gong and Y. Yang, The 18^th National Conference on Electrochemistry. August, 2015, Haerbin, China.