Edited by: Jiexi Wang, Central South University, China
Reviewed by: Hong Guo, Yunnan University, China; Xiaobo Ji, Central South University, China
*Correspondence: Hongjun Yue
This article was submitted to Physical Chemistry and Chemical Physics, a section of the journal Frontiers in Chemistry
†These authors have contributed equally to this work.
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Li/CFx is one of the highest-energy-density primary batteries; however, poor rate capability hinders its practical applications in high-power devices. Here we report a preparation of fluorinated graphene (GFx) with superior performance through a direct gas fluorination method. We find that the so-called “semi-ionic” C-F bond content in all C-F bonds presents a more critical impact on rate performance of the GFx in comparison with sp2 C content in the GFx, morphology, structure, and specific surface area of the materials. The rate capability remains excellent before the semi-ionic C-F bond proportion in the GFx decreases. Thus, by optimizing semi-ionic C-F content in our GFx, we obtain the optimal x of 0.8, with which the GF0.8 exhibits a very high energy density of 1,073 Wh kg−1 and an excellent power density of 21,460 W kg−1 at a high current density of 10 A g−1. More importantly, our approach opens a new avenue to obtain fluorinated carbon with high energy densities without compromising high power densities.
Fluorinated carbon (CFx) possesses a very high theoretical energy density (2,180 Wh kg−1 when x equals 1 for fluorinated graphite) as a cathode material for primary lithium batteries, thus has been strongly desired in many civil and military applications that require a long service-life, wide range of operating temperatures, as well as high energy densities and reliability. Fluorinated graphite has been widely investigated (Nakajima et al.,
Coating of highly conductive materials, such as carbon, polypyrrole, and polyaniline on the surface of carbon fluorides is helpful to improve the rate capability (Zhang Q. et al.,
Fluorinated graphene, as a two-dimensional (2D) material, can shorten the diffusion path of lithium ions, which is helpful for rapid transfer of lithium ions (Zhang S. S. et al.,
In this study, a fluorinated multilayered graphene (GFx) was prepared by a direct gas fluorination of RGO instead of graphene oxides (Damien et al.,
Fluorinated graphenes were prepared by a one-step gas-phase fluorination of RGO as described in previous work (Yue et al.,
X-ray powder diffraction (XRD) technique was employed to characterize phases of as-prepared materials, using Cu Kα radiation (1.54178 Å) on a Miniflex600 (Rigaku, Japan) instrument. XRD patterns were collected with a step of 0.0167°, and 20 s per step. 13C and 19F magic angle spinning (MAS) NMR experiments were performed on Bruker 600 MHz AVANCE III spectrometer using Hahn-echo pulse under the spinning frequencies of 12 and 60 kHz, respectively. Recycle delays of 60 and 20 s were applied for complete relaxation of excited magnetization for the acquisition of quantitative 13C and 19F NMR spectra. The chemical shifts of 13C and 19F were referenced to diamantine (38.6 ppm) and LiF (−204 ppm). X-ray photoelectron spectroscopy (XPS) of the samples was measured by an ESCALAB 250Xi spectrometer (Thermo Fisher). SEM images were performed on scanning electron microscopy (SEM) (ZEISS). The transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) analysis were performed on Tecnai F20 (FEI, US), operating at 200 kV. Nitrogen adsorption/desorption isotherms and Brunauer-Emmett-Teller (BET) surface area were performed on a Quantachrome instrument Autosorb-iQ.
The cathode was prepared by mixing 80 wt.% fluorinated graphene, 10 wt.% acetylene black, and 10 wt.% poly (vinylidene fluoride) (PVDF). Aluminum disks were employed as current collectors and the active materials on the Al disks are between 1.5 and 2 mg cm−2. A lithium metal disk was used as a counter electrode, and electrolytes were 1 M LiPF6 dissolved in ethylene carbonate/dimethyl carbonate (1:1 volume ratio). Discharge tests were performed at various currents with a cutoff voltage of 1.5 V by a LAND CT2001A battery test system at 25°C.
It is well known that the fluorine content in fluorinated carbon significantly affects the electrochemical performance. A low fluorine content results in a good power density but relatively low energy density. To achieve excellent power densities with high energy densities, we prepared fluorinated graphene (GFx) with various F content and investigated the factors influencing the electrochemical performance. The F content in the GFx was controlled by varying the temperatures during the fluorination; for example, GF0.5, GF0.8, and GF1.1 were obtained at 400, 430, and 460°C, respectively.
When the x in GFx equals 0.5, as shown in Figure
Selected discharge curves of
With the impressive rate capability, the specific capacity of the GF0.5 is yet to be satisfied due to its low theoretical capacity (623 mAh g−1). In contrast, the theoretical capacity of the GF1.1 is up to 896 mAh g−1. As shown in Figure
The relatively poor rate capability of the GF1.1 is associated with its poor electrical conductivity, which needs to be addressed. Therefore, the way to gain high power densities while retaining high energy densities is to prepare CFx with high F content without compromising the electrical conductivity. To achieve this goal, instead of with complicated surface treatments (Groult et al.,
To understand the electrochemical performance of GFx electrode, we compared the kinetic properties of GF0.5, GF0.8, and GF1.1 by electrochemical impedance spectroscopy (EIS) measurement. To exclude the effect of conductive carbon, we prepared the electrodes with the GFx materials without addition of carbon black. In the absence of conductive carbon, however, the impedances of the fresh electrodes were too extremely high to obtain accurate results for comparison (Figure
Ragone plots were employed to depict the advanced electrochemical performance of the GF0.8 (Figure
As mentioned above, we achieve high energy densities with excellent power densities by the full use of the advantage of semi-ionic C-F bonds in the CFx. Ionic, semi-ionic, and covalent C-F bonds are the three types of C-F bonds in the CFx. Ionic C-F bonds are typically only formed when x in CFx is very small (e.g., <0.05 for graphite; Amine and Nakajima,
In contrast, semi-ionic C-F bonds are essentially covalent, with which, however, the conjugated C-C bonds are preserved between carbon atoms unbounding to fluorine with the F-C-C angle of 90° and the neighboring C-C bond length of ~0.14 nm (Sato et al.,
19F NMR spectra were employed to distinguish covalent and semi-ionic C-F bonds. Figure
The semi-ionic C-F bond ratios in the GFx were determined by fitting of 19F NMR spectra (Figure
Another possible reason for poor electrical conductivity related to the low semi-ionic C-F bond ratio is the high F content resulting in low content of the sp2 hybridized C. Figure
Based on the 19F and 13C NMR analysis, we can conclude that with the increasing F content in GFx, the sp2 C ratio decreases while the semi-ionic C-F bond ratio remains unchanged until the critical x of 0.8, beyond which the electron-transfer ability of sp2 C is compromised. This matches very well the electrochemical performance of GF0.5, GF0.8, and GF1.1, namely, the GF0.5 and GF1.1 depicted high power densities and high capacities, respectively, but the GF0.8 exhibited the optimal electrochemical performance (21,460 W kg−1 and 1,073 kWh kg−1).
Semi-ionic C-F bond ratios in CFx also have been analyzed using X-ray photoelectron spectroscopy (XPS) spectra (Doniach and Sunjic,
XPS C1s spectra of
In contrast, the semi-ionic C-F bonds were barely detected by XPS C1s spectra for the GF0.8 and GF1.1 (Figures
Besides the semi-ionic C-F bond ratio, other factors that may influence the electrochemical performance of the GFx were also investigated, including structure, morphology, and surface area.
XRD patterns of fluorinated graphene materials were shown in Figure
Figure
SEM images of
During fluorination, the surface area may change, thereby contributing to the improved rate performance. Therefore, the specific surface areas of the pristine RGO and the fluorinated graphene materials were analyzed using Brunauer–Emmett–Teller (BET) method. Figure
After fluorination, the specific area increased, which is consistent with the SEM observation, in which the fluorinated graphenes exhibit more lamellar architectures, compared with the pristine RGO (Figure
Fluorinated graphenes were prepared using one-step gas fluorination of RGO at elevated temperatures. The impacting factors, including semi-ionic C-F ratio, sp2 C content, structure, morphology, and specific surface area are investigated to gain fluorinated graphenes with high power densities and high energy densities. The semi-ionic C-F ratio in the fluorinated graphene shows the most critical influence on achievement of high rate performance. Thus, by manipulating the semi-ionic C-F proportion in the fluorinated graphene by temperature control, we obtain the optimal x of 0.8 in GFx; the GF0.8 exhibited a high energy density of 1,073 Wh kg−1 and an excellent density of 21,460 W kg−1 at a high current density of 10 A g−1 (about 12C rate). Compared with those using additional steps (such as C coating and hydrothermal treatment) to improve the rate performance of as-obtained CFx, we offer a one-step approach to obtain high energy densities without compromising power densities for preparation of fluorinated carbon, showing very promising practical application.
The work cannot be completed without kind cooperation of all authors. GZ: Acquired and analyzed the NMR and XPS data; HC: Carried out the material preparation and electrochemical test; XH and HC: Carried out and analyzed the SEM, TEM, and BET analysis; GZ and XH: Wrote the paper and all authors discussed the results and revised the manuscript; HY, GZ, HC, and XH: Proposed the research; HY and CL: Attained the main financial support for the research and supervised all the experiments.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The reviewer XJ, and handling Editor declared their shared affiliation.
The study was supported by the National Natural Science Foundation of China (21503232 and 21603231); Fujian Natural Science Foundation of China (2060203); and Xiamen Science and Technology Planning Project of China (3502Z20161246 and 3502Z20172030).
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