Investigation of the Reaction Mechanism of Graphite Fluoride Rechargeable Batteries
Authors: Tomioka Shuta , Tsujimoto Shota , Lee Changhee , Miyahara Yuto , Miyazaki Kohei , Abe Takeshi 
Published: December 2024

Introduction

With the accelerated development of next-generation rechargeable batteries, graphite fluoride with high energy density and excellent discharge potential flatness into rechargeable batteries has recently become to attract attention as an active material. In particular, lithium-graphite fluoride rechargeable battery using graphite fluoride derived from natural graphite has been reported in a previous study, which has increased the feasibility of this system.1 The charge and discharge reactions of this system can be considered as follows.2

(CxF)n + nLi+ + ne- ⇆ xnC + nLiF (1)
In charging process, carbon is fluorinated and graphite fluoride is formed. However, it has problems such as extremely fast degradation and increased potential hysteresis compared to lithium-ion batteries. These problems may be caused by the low conductivity and high dissociation energy of lithium fluoride. Another possible cause is the amorphous carbon formed when graphite fluoride is discharged, which is unfavorable for graphite fluoride compounds (F-GICs) formation. However, no detailed studies about F-GIC forming mechanism have been carried out, and further research is required.
In this study, experiments were carried out using graphite fluoride composite electrodes. Furthermore, we used the composite electrode mixed with carbon and lithium fluoride as a model of discharge product of graphite fluoride. From these experiments, the aim is to clarify the electrochemical reaction mechanism of this system by investigating the influence of the size of lithium fluoride and the crystallinity of carbon on the electrochemical properties.

Experimental methods

First, in order to investigate the influence of lithium fluoride crystal size, graphite fluoride (CF) obtained by fluorinated scary graphite composite electrodes were prepared. The electrodes were first discharged to 1.5 V at a current density of 50 mA g-
1. Then, the electrodes were left at OCV for 1 hour or 1 week to prepare electrodes with different crystallinity of lithium fluoride.
Second, in order to investigate the influence of the crystallinity of carbon, ball-milled and non-ball-milled natural graphite (SNO-15) were prepared and mixed with lithium fluoride, respectively, to produce carbon and lithium fluoride composite electrodes named WBMCL and WNBMCL electrodes, respectively.
Two-electrode type coin cell was used for the electrochemical measurements. The composite electrodes were used as the working electrode and lithium metal was used as the counter electrode. The electrolyte was 5.8 mol dm-
3 LiBF4/sulfolane (C4H8O2S). Charge-discharge measurements were carried out at a current density of 50 mA g-
1 and in a voltage range of 1.5-5.3 V.

Results and discussion

(Effect of LiF size)
XRD measurements were performed on cells left at OCV for 1 hour and 1 week after the first discharge of the CF composite electrode, showing that lithium fluoride crystal growth was more advanced on the electrode left for 1 week after discharge. To investigate the effect of lithium fluoride crystal size, charge-discharge experiments were carried out using these electrodes with different lithium fluoride crystal sizes. Charge-discharge curves are shown in Figure 1 (a). It was confirmed that there was not significant difference in reversible capacity between these two electrodes, and that the size of the lithium fluoride had no adverse effect on the charging of the lithium-graphite fluoride rechargeable battery. This result indicated the fact that partially dissolved fluoride ions from lithium fluoride react with carbon, rather than directly dissociating and reacting with lithium fluoride, which has a high dissociation energy.
(Effect of crystallinity on the system)
XRD measurements confirmed that ball-milled carbon was amorphous. To investigate the effect of carbon crystallinity, charge-discharge measurements of WBMCL and WNBMCL electrodes were carried out. Results are shown in Figure 1(b), (c). It was confirmed that the WBMCL composite electrode had a larger capacity. From these results, it can be inferred that graphite with high crystallinity is not suitable for charging lithium-graphite fluoride rechargeable batteries, and that amorphous carbon is more advantageous.
In summary, on the charge-discharge reaction of lithium-graphite fluoride rechargeable batteries, the effect of carbon crystallinity was found to be more significant than the crystallite size of lithium fluoride.

References

1) Ito, C. Lee, Y. Miyahara, S. Yamazaki, T. Yamada, K. Hiraga, T, Abe, and K. Miyazaki, Chem. Mater., 34, 8711-8718, (2022).
2) Zhan, S. Yang, Y. Wang, Y. Wang, L. Ling, K, and X. Feng, Adv. Mater. Interfaces., 1, 4, (2014).

Figure 1