Facile Synthesis of Solvent Sensitive Fluorinated Graphene Nanoscrolls and Application as a Cathode Material for Lithium Ion Batteries

In this work, fluorinated graphene nanoscrolls (FGN) were synthesized via facile chemical methods under simple and mild conditions. Interestingly, the formation of the featured FGN was significantly solvent sensitive. Experimental results indicated that in the presence of aprotic solvent, for example, N,N dimethylformamide (DMF), the reaction system inclined to form the interesting FGN nanostructures. The structure and morphology of the prepared FGN were detailed characterized by atomic force microscopy (AFM), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray diffraction (XRD) etc. The obtained FGN was used as a cathode material for primary lithium ion batteries with superior discharge specific capacity (eg. 979.3 mAhg-1), stable discharge platform and high energy density (eg. 2287.9 Wh kg-1), which fosters it a high density, low cost and durable candidate for cathode material for lithium ion batteries..


Introduction
With the digital development of military and aviation technology (for example, satellites, spacecraft, space stations, deep space exploration landers, orbit transfer vehicles and drones etc.) [1][2] , the requirements for various high-performance power sources are getting higher and higher [3][4] . For example, the weight of each component of the power sources was required to be as light as possible to increase the effective load, the power supply was also demanded to be able to discharge at a large current and provide higher power to meet the rapid response of military or aviation equipment [5] .
Lithium/carbon fluoride (Li/C1Fx) primary battery is a kind of solid cathode lithium battery first commercialized [6] , and its theoretical mass specific energy can achieve to 2180Wh/kg [7] , which was much higher than widely used Li/SOCl2 primary battery (1470Whkg -1 ) [8] and Li/MnO2 candidate (1005Whkg -1 ) [9] . The featured lithium/carbon fluoride primary battery has the advantages of a stable discharge platform, excellent safety and storage performance, together with wide operating temperature range [10][11] . However, due to the poor intrinsic conductivity of the cathode material fluorocarbon and the slow electrode dynamics process, the lithium/carbon fluoride primary battery has a larger polarization performance during the discharge processes. Meanwhile, the layered structure of the described lithium/carbon fluoride is not conducive to the lithium ion electrochemical reaction [12] . Therefore, the lithium/carbon fluoride battery is limited to discharge at a low rate, leading to voltage hysteresis at the beginning of the discharge. At the same time, there is a large amount of heat consumption during the discharge process of Li/C1Fx primary battery, which affects the performance of the battery obviously and severely restricts the application of lithium/carbon fluorine batteries [13] .
Facing such difficulties, necessary modification for the lithium/carbon fluoride cathode material to enhance the electrochemical performance was expected. In the recent year, different attempts, for example, MnO2 doped Li/C1Fx [9] , multiple-walled carbon nanotubes modified Li/C1Fx [14] , polyaniline coated Li/C1Fx [15] , conductive polymer coated Li/C1Fx [16] and modification to fluorinated graphene [17] etc., were commonly used as electrode material to improve the rate performance of fluorinated carbon materials in lithium batteries. Among which, significant attention had paid to the electrode material of fluorinated graphene [18] , which was derived from C1Fx. The electrochemical performance of the derived fluorinated graphene was significantly better than the C1Fx candidate. This was mainly due to the fact that the edge and base surface of the cathode electrode material of fluorinated graphene contains a large number of C-F covalent bonds after modification to fluorinated graphene. Therefore, when the modified fluorinated graphene was used as cathode materials of lithium batteries, the lithium ion diffusion rate was significantly improved [19] . Importantly, the superiority of stable discharge platform, excellent safety and storage performance and wide operating temperature range of the Li/C1Fx electrode material was reserved.
Until now, few report had paid to the preparation of fluorinated graphene nanoscrolls (FGN) or nanorods and corresponding application as a cathode material for lithium ion batteries. In this work, a novel FGN nanostructure with different fluorine doping contents were prepared successfully from fluorinated graphite (C1Fx) substituted with different fluorine contents by modified Hummers method. Interestingly, the FGN prepared in this work exhibited interesting solvent regulation characteristics. In aprotic solvents, the featured FGN can obtain readily in the reaction system. While in protonic solvents, the reaction system inclined to formation of nanosheets structured fluorinated graphene instead of the featured FGN. Meanwhile, further investigation on FGN as cathode material for primary lithium batteries was also carried out. Investigations revealed that the solvent-regulated FGN prepared in this work exhibited excellent electrochemical characteristics and large power density when used as a cathode electrode material for primary lithium batteries.

Results and discussion Preparation and Characterization of Solvent Sensitive FGN
To date, few reports had been demonstrated on the FGN preparation and application, which might introduced C-F semi-ionic bonds and a large number of defects on the basis of pristine graphene to accelerate the conduction of lithium ions and promote the transmission of ions and electrons as a new high-efficiency lithium battery cathode material. In this work, we used graphite fluoride (C1Fx) as the starting material. By using modified Hummers method [20][21] , fluorinated graphene oxide (FGO) nanosheets with abundant oxygen-containing groups was obtained. The obtained FGO intermediate was reduced by hydrazine hydrate under reflux conditions, giving commonly reported fluorinated graphene nanosheets (Scheme 1) [22] .
Interestingly, the formation of the prepared fluorinated graphene nanostructure showed significantly solvent sensitive characteristics. Experimental results indicated that in the presence of protic solvent, for example, ethanol, methanol or water etc., wrinkled fluorinated graphene nanosheets with a thickness of about 0.9 nm were given readily (determined by atomic force microscopy (AFM) analysis, Fig. 1a). While when the obtained wrinkled fluorinated graphene nanosheets were re-dispersed in aprotic solvent, for example, N,N dimethylformamide (DMF), dimethyl sulphoxide (DMSO) or N-methylpyrrolidone (NMP) etc., by further ultrasound for 15 min and aging for 24-72h, the reaction system inclined to form the interesting FGN nanostructures (  The obtained FGN nanocomposites were also characterized by XRD etc. From the XRD patterns showed in Fig. 2, we can observe that the prepared FGN nanocomposites gave a characteristic peak at 2θ =12.9°, which was in conformity with literature results of fluorinated graphene peak (001) [20] , confirming that the prepared FGN nanocomposites possessed the structure characteristics of fluorinated graphene. Peak at 2θ =42.0° can be assigned the graphene peak (100) (corresponding to C-C bonds in graphene structure [22] ), indicating that the prepared FGN nanocomposites were partially restored to ordered crystal structure of graphene.

Possible mechanism for the formation of Solvent Sensitive FGN
The possible mechanism for the featured FGN nanocomposites formation could be described by the procedures showed in Scheme 1. Firstly, FGO nanosheets were prepared by modified Hummers method (step 1). In this work, that graphite fluoride with different fluorine substituted ratios was used as the starting material. As shown in Scheme 1, after reduction treatment illustrated in step 2 of Scheme 1, fluorinated graphene nanosheets with 2D nanosheet morphology was obtained readily.
Due to containing of abundant C-F and C-H bonds, the prepared fluorinated graphene nanosheets described in Scheme 1 evoked abundant hydrogen bonds. Meanwhile, the prepared fluorinated graphene nanosheets held strong π-π interaction with the special fused aromatic structure or benzene rings in the the same or different nanosheet. It's noteworthy that when such hydrogen bonds force were occurred in aprotic solvent, for example, DMF, dimethyl sulfoxide (DMSO) and N-methyl pyrrolidone (NMP) etc. Due to deficiency of protons in the solvent, the hydrogen bonds in fluorinated graphene inclined to form in the same fluorinated graphene nanosheet or multiple fluorinated graphene nanosheets (between F atoms and H atoms, further multiple effects led to formation of the nanoscroll structure finally) instead of hydrogen bonds between fluorinated graphene nanocomposites and solvent moleculars. While when protonic solvent, for example, H2O, ethanol, methanol etc., each fluorinated graphene nanosheet was surrounded by many protonic solvent moleculars (e. g. H2O), besides hydrogen bonds in the same fluorinated graphene nanosheet or different ones, abundant hydrogen bonds between solvent moleculars and fluorinated graphene nanosheets would form, which can drive the prepared nanocomposites to form the commonly nanosheet structured fluorinated graphene. Moreover, synergistic effects of plentiful hydrogen bonds, π-π interactionand and electrostatic interactions in singled or multiple nanosheets during the the SCI formation processes (step 3) would further accumulate and accelerate, leading to heavily rolled structure of intermediate SCI. After aging treatment for 24-72h, the featured FGN nanocomposites could be obtained readily (step 4 of Scheme 1). In such procedures, associated self-assembly procedures would also occur, which was helpful for the formation of FGN nanocomposites (Scheme 1).
Further investigations confirmed the solvent sensitive characteristics of the prepared fluorinated graphene. As shown in TEM image of Fig. 3a, when the prepared nanocomposites was dispersed in ethanol, a typical protonic solvent, due to multiple forces described above, the reaction system formed nanosheet structured fluorinated graphene. On a comparison study, when aprotic solvent, for example, DMF, DMSO etc, was used, nanoscrolls structured fluorinated graphene instead of nanosheet structured one, can be given, which can be observed by TEM images (Fig. 3b-c). Interestingly, when nanoscrolls structured FGN showed in Fig.3b was re-dispersed in ethanol, as examined by TEM image (Fig. 3d), the reaction system inclined to shift back to nanosheet structured fluorinated graphene, clearly exhibiting the interesting solvent sensitive fluorinated graphene nanocomposites.

Electrochemical performance of the prepared FGN as a cathode material for lithium ion batteries
The electrochemical performance of the prepared FGN as a cathode material for lithium ion batteries was tested in this work. Fig. 4 showed the constant current discharge curve of the prepared FGN with different fluorine substitution ratios as the cathode electrode material, which was then assembled into a button battery. As shown in Figure 4a-b, when FGN-1.1 or FGN-0.8 was used, the increase in discharge current density led to a decrease in average discharge voltage and specific capacity. This should be caused by the Ohmic polarization effect. At the same time, we can also observe intuitively that the electrochemical properties of the prepared FGN were closely related to the fluorine content. Under the same current density, the discharge specific capacity was directly proportional to the fluorine content of the prepared FGN. For example, FGN-1.1, the specific discharge capacity of FGN-1.1 was measured at three different current densities with below values: 979.3 mAhg -1 (50mA), 802.6mAhg -1 (100mA), 670.7mAhg -1 (200mA) (Fig. 4a), which was much higher than the corresponding discharge specific capacity values when FGN-0.8 was determined: 902.8 mAhg -1 (50mA), 765.1mAhg -1 (100mA) and 648.2.9 mAhg -1 (200mA) (Fig. 4b). This might on account of that FGN with higher fluorine content possessed lower conductivity, resulting in a larger Ohmic polarization effects and a decrease of discharge voltage. What's more, experimental results indicated that the prepared FGN nanocomposites, eg. FGN-1.1, had stable discharge platform range from 50-600 mA. And the discharge voltages of FGN-1.1 were relatively lower than those of lower fluorine substituted FGN (for example, FGN-0.8). As shown in Fig. 4a (FGN-1.1), the discharge voltages were 2.42 (50mA) and 2. 19 (200mA), respectively, which were also obviously higher than the contradistinctive data showed in Fig. 4b (FGN-0.8, 2.68 (50mA), 2.36 (200mA)). Detailed discharge performances of different fluorine substituted FGN revealed that the highest discharge specific capacity (979.3 mAhg -1 ) can be achieved when the fluorine substitution ratio of FGN was 1.1 at the current density of 50mA.  On the other hand, a further comparative energy density analysis of of the prepared FGN were carried out in this work. As shown in Fig. 5, due to decrease of discharge voltage and specific capacity, the energy density of the FGN decreased as the power density increased. It's noteworthy that at low energy density, the prepared FGN (x=0.8) showed good electrochemical performance, and the energy density reached the maximum value (2287.9 Wh kg -1 ) at a current of 50 mAg -1 . It is gratifying that the measured value was significantly higher than the theoretical energy density of the Li/C1Fx galvanic cell mentioned above (2180 Wh kg -1 ), indicating that modification of carbon fluoride (C1Fx) to fluorinated graphene led to significantly improved of energy density. Meanwhile, at high value of Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 19 October 2020 doi:10.20944/preprints202010.0383.v1 energy density, the prepared FGN with higher fluorine contents (x=1.1 instead of 0.8) exhibited higher energy density. As shown in Fig. 5, when the current was 1250mAg -1 , the energy density can even reach 1613.8 Wh kg -1 , which was obvious higher than the value (1416.5 Wh kg -1 ) of FGN with lower fluorine contents (x=0.8) under the same current density.

Materials and characterization
Graphite fluoride with different fluoride contents (99.99%, 325 mesh) were purchased from Alfa Aesar. Hydrazine hydrate (analytically pure) was purchased from Sigma Aldrich. All solvents and other reagents were purchased from Beijing Chemicals Co. Ltd. as analytical-grade products.
The powder X-ray diffraction (XRD) measurements of the samples were recorded on a Bruker D8-Advance X-ray powder diffractometer using Cu Kα radiation (λ=1.5406 Å) with scattering angles (2θ)

Typical procedures for the preparation of FGN
Fluorinated graphene oxide (FGO, showed in Scheme 1) was prepared by modified Hummers method [21][22] . In a typical procedures, graphite fluoride powder (1.5 g, 325 mesh, C1Fx, x=0.5, 0.6, 0.8 and 1.1 etc.) was put into a mixture of 12 ml concentrated H2SO4, 2.5 g K2S2O8 and 2.5 g P2O5. The solution was heated to 80 o C and kept stirring for 5 h by using oil-bath. In a next step, the mixture was cooled to room temperature and diluted with deionized water (500 ml) overnight. Then, the product was obtained by filtering using 0.2 micron nylon film and dried naturally. The pre-oxidized graphite was then re-oxidized by Hummers method. Pretreated graphite powder was put into 0 o C concentrated H2SO4 (120 ml). Then, 15 g KMnO4 was added gradually under stirring and the temperature of the mixture was kept to be below 20 o C by ice-bath. Successively, the mixture was stirred at 35 o C for 4 h, and then diluted with 250 mL deionized water by keeping the temperature under 50 o C. 700 ml deionized water was then injected into the mixture followed by adding 20 ml 30% H2O2 drop by drop. The mixture was filtered and washed with 1 : 10 HCl aqueous solution (1 L) to remove metal ions followed by 1 L of deionized water to remove the acid. The resulting solid was dried in air and diluted to make FGO dispersion (0.5% w/w). Finally, it was purified by dialysis for one week to remove the remaining metal species, finally, the sample of FGO was obtained by filtration and dried in vacuum for 24 h at 60 o C. Successively.
Firstly, the sample of FGO was re-dispersed in water (0.5 mg/ml) and ultrasoniced for 15 min, then hydrazine hydrate (1 ml) was added gradually to the mixture. The suspension was stirred at 80 o C for 24 h. Finally, the dispersion was cooled down and filtered, the solid sample was collected after thoroughly washing with deionized water and ehanol until the impurities can be removed completely. The sample was vacuum-dried at 38.6 o C for 24 h, affording the reduced composite: nanosheet structured fluorinated graphene (detected by AFM, showed in Fig. 1a) can be afforded.
On a comparison study, the obtained solid of FGO was re-suspended in DMF to form a dispersion of 1 mg/ml. The mixture (100ml) was ultrasoniced for 15 min, then hydrazine hydrate (1 ml) was added gradually to the mixture. The suspension was stirred at 80 o C for 24 h. Finally, the dispersion was cooled down and filtered, the solid sample was collected after thoroughly washing with deionized water and DMF until the impurities can be removed completely. The sample was vacuumdried at 38.6 o C for 24 h, affording the reduced composite: FGN(detected by TEM analysis, Fig. ) .

Electrochemical analysis
In this work, after preparation of button half-cells, cyclic voltammetry (CV) test was used to study the surface chemical reactions of the prepared electrodes during charge and discharge procedures, so that to explore the mechanism of electrodes reaction. The CV scan voltage range was 0.01-3.0V, and the scan rate is 0.5mVs -1 . Meanwhile, the capacity, coulombic efficiency, cycle and rate characteristics of FGN were studied through constant current charge and discharge tests.

Conclusion
In this paper, a novel fluorinated graphene nanoscrolls (FGN) with different fluorine doping contents were synthesized via facile chemical methods under simple and mild conditions. Interestingly, the formation of the featured FGN was significantly solvent sensitive. Experimental results indicated that in the presence of aprotic solvent, for example, N,N dimethylformamide (DMF), the reaction system incline to form the interesting FGN nanostructures. The structure and morphology of the prepared FGN were detailed characterized by AFM, TEM, SEM and XRD etc. Further investigation on FGN as cathode material for primary lithium batteries was also carried out. Investigations revealed that the solvent-regulated FGN prepared in this work had excellent electrochemical characteristics and large power density when used as a positive electrode material for primary lithium batteries. Detailed discharge performances of different fluorine substituted FGN indicated that the highest discharge specific capacity (979.3 mAhg -1 ) and stable discharge platform range from 50-600 mA were observed when the fluorine substitution ratio of FGN was 1.1 at a current density of 50mA. On the other hand, FGN with lower fluorine content (the fluorine substituted ratio was 0.8 instead of higher ones) can reach the maximum energy density (2287.9 Wh kg -1 ) at a current of 50 mAg -1 , which fosters it a high density, low cost and durable candidate for cathode material for lithium ion batteries.
Data availability. The data that support the plots within this paper are available from the corresponding author upon reasonable request.