Amorphous, Highly Disordered Carbon Fluorides as a Novel Cathode

Oct 18, 2016 - Shanghai Key Laboratory of Molecular Catalysts and Innovative Materials, Department of Chemistry, Fudan University, 220# Handan Road, ...
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Amorphous, Highly Disordered Carbon Fluorides as a Novel Cathode for Sodium Secondary Batteries Wen Liu,† Yong Li,† Bin-Xin Zhan,† Bin Shi,† Rui Guo,† Hai-Juan Pei,† Jing-Ying Xie,*,† and Zheng-Wen Fu‡ †

Shanghai Institute of Space Power-Sources, 2965# Dongchuan Road, Shanghai 200245, P. R. China Shanghai Key Laboratory of Molecular Catalysts and Innovative Materials, Department of Chemistry, Fudan University, 220# Handan Road, Shanghai 200433, P. R. China

‡

S Supporting Information *

ABSTRACT: A strategy for enabling a novel carbon fluoride (CFx) to be a highperformance cathode material for sodium batteries is proposed and realized. An amorphous and highly disordered CFx, denoted as d-CFx, is prepared through a facile synthesis route. Herein we report on a Na/d-CFx cell reversibly discharging/ charging at such a low overpotential and capacity-decay rate. The polarization of the Na/d-CFx cell is about 780 mV, the lowest value reported in the state-of-the-art Na/CFx system. The initial discharge capacity is 582.5 mA h/g, with a capacity of 412.5 mA h/g after 12 cycles. The designed d-CFx improves the Na+ diffusion rate and facilitates the reaction with larger sodium ions during the discharging process. Amorphous discharge products NaF are easily decomposed, enhancing the charge reaction. The small grain size of NaF is believed to be another key factor that enables the minimal charge gap of Na/d-CFx cells and a slower capacity-decay rate, which can facilitate the decomposition of NaF and the reformation of C−F bonds during the charging process. Generally speaking, this study encourages the further exploration of secondary sodium/CFx batteries, and the cyclability of the sodium anode is included.

1. INTRODUCTION Carbon fluorides (CFx) were first introduced as a cathode in primary lithium batteries for their highest energy density among the solid cathode systems.1 During the discharging process, most agree that, to a certain degree, lithium ions enter carbon fluoride solvated with the nonaqueous electrolyte solvent, with the formation of a transient compound of Li+(solvent)CF, which spontaneously decomposes to C + LiF + solvent.2 The electrochemical reaction of Li and CFx was validated to show no evidence of its reversibility,3 which was attributed to the high electromotive force needed to decompose discharge product LiF.4 Especially deserving to be mentioned, the CFx cathode shows a rechargeable capability in sodium ion batteries.4−6 Shao6 and coauthors have systematically unveiled the conversion reaction mechanism involved in the charging and discharging processes, both on the surface and in the bulk, and the results show evidence of reversible conversion between CFx and NaF in the Na/fluorinated carbon fiber (CF0.75) system. During the discharging process, sodium ions diffuse into the bulk of the material and generate NaF in the layer space of the carbon structure, resembling the mechanism of Li/CFx batteries. Sodium/carbon fluoride (Na/CFx) batteries exhibit some significant advantages: high energy density and low cost (abundant resources sodium in nature).4 However, Na/CFx batteries suffer from large polarization during the charging/ discharging process, especially the polarization during the © XXXX American Chemical Society

charging process. Additionally, another drawback of low cycle stability still needs to be overcome for practical applications. Because the Na/CFx secondary battery system is being reported, much work has being conducted to address these issues, such as introducing catalysts to decrease the charge potential, the designation of the core−shell structure to improve the cycle life, and using a partially fluorinated carbon fiber to enhance the discharge voltage and cycle performance.5,6 In Na/fluorinated multiwalled carbon nanotube (FMWCNT) batteries,5 MWCNTs formed after the discharging process with an intrinsically high surface area, high catalytic activity, and cage-type structure enhancing the charge reaction activity. The polarization of the Na/F-MWCNT battery was 1600 mV, about 400 mV lower than that of Na/commercial CFx batteries. The combination of graphene (GNS) and FeF3 with CFx provides more active sites to facilitate the decomposition of NaF during the charging process.5 And the CFx/GNS/FeF3 (CGF) electrode may have specific channels to avoid the dissolution of fluorine into the electrolyte. Moreover, the high electronegativity of the F atom in FeF3 might weaken the bonding energy of NaF during the charging process and lower the energy needed to decompose NaF, leading to a voltage gap of only 800 mV. In the Na/CFx system, fluorinated Received: July 16, 2016 Revised: September 21, 2016 Published: October 18, 2016 A

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Figure 1. (a) SEM and (b) TEM images of CFx. (c) SEM and (d) TEM images of d-CFx.

carbon fiber with a special three-dimensional disordered carbon structure and different kinds of metastable structures delivered an initial discharge capacity of 705 mA h/g with the highest discharge plateau of 2.75 V at 20 mA/g.6 But the discharge capacity was only about 250 mA h/g after 10 cycles. To the best of our knowledge, none of these achievements can meet practical needs. As the conversion reaction mechanism involved in the charging and discharging process of fluorinated carbon fiber with sodium was disclosed,6 evidencing the reversible conversion between CFx and NaF, an emphasis should be placed on employing more novel fluorinated carbon materials to improve the electrochemical performance of Na/CF x batteries. Amorphous materials exhibit isotropy in macrostructure and own abundant defects, which guarantees and facilitates the lithium ion7−9,10,11 or sodium ion,12−15 even multivalent ions including Mg2+ and Al3+,16−18 transferring from the electrolyte to the interior of the materials. On the other hand, the amorphous structure can buffer volumetric expansion during the charge−discharge cycling process,19,20 leading to enhanced cyclic performance. Amorphous or disordered material would be an excellent candidate for the realization of enhanced performance in the Na/CFx system. In this study, we utilized commercial CF x to synthesize amorphous and highly disordered CFx (dubbed as d-CFx) as a cathode for sodium batteries with the lowest voltage gap (780 mV) and capacitydecay rate.

monochromatized source of Cu Kα radiation (k = 0.1540 nm) at 1.6 kW. Raman spectra were obtained with a Labram HR800 spectrometer (HORIBA Jobin Yvon) by using an excitation wavelength of 532 nm. Scanning electron microscope (SEM) images were captured using a field emission scanning electron microscope (FE-SEM, Hitachi S-4800) operated at an acceleration voltage of 10 kV. Transmission electron microscope (TEM) and selected area electron diffraction (SAED) measurements were carried out with a 200 kV side entry JEOL 2010 TEM. X-ray photoelectric spectroscopy (XPS) analysis was carried out on an ESCALAB 250Xi system (Thermo) with a monochromatic Al Kα X-ray source. To avoid the signal of fluorine from PVDF, the preparation of the d-CFx electrode for XPS measurement was different from that for electrochemical measurement, in which the d-CFx powders were dispersed in NMP and cast on the Ni foam, and then the electrode was placed in an oven for drying at 60 °C for 24 h. For the ex situ measurements, to avoid exposure to oxygen or water, care must be taken in the handling of samples in different states. The cells were disassembled in an Ar-filled drybox, and the electrodes were rinsed in anhydrous DME to eliminate residual salts; the active materials were rapidly transferred into the chambers in a few seconds. 2.3. Electrochemical Characterization. The working electrodes were composed of active material (d-CFx, 80 wt %), an electrical conductor (vapor-grown carbon fiber (VGCF), 10 wt %), and binder (poly(vinylidene fluoride) (PVDF), 10 wt %) on titanium foil. One sheet of high-purity sodium foil (99.9%) was used as the anode, 0.25 M NaPF6/1,2-dimethoxyethane (DME) was used as the electrolyte as described in our previous work,4,5 and Celgard 2325 was used as the separator. The 2016-type coin cells were assembled in an argon-filled glovebox. Galvanostatic charge−discharge measurements were carried out at room temperature with a Land CT 2001A battery test system. Cutoff voltages were 1.5 and 4.5 V. The current densities and capacities of electrodes were calculated on the basis of the weight of active material. The average mass of active d-CFx on one electrode was about 0.8−1.0 mg.

2. EXPERIMENTAL SECTION 2.1. Synthesis of d-CFx Material. d-CFx was prepared through a simple route. First, 50 mg of CFx (x ≈ 0.66, Gary, Zhuoxi Technology Co., Ltd., China) was dispersed in 400 mL of NMP with vigorous stirring. The suspension was heated to 90 °C and maintained at this temperature for 8 h before it was cooled to room temperature. The resulting solution was ultrasonicated for 30 min before it was dried at 80 °C to obtain the d-CFx material (black). 2.2. Structural Characterization. The as-prepared d-CFx material and the discharge/charge products were characterized by X-ray diffraction (XRD, Bruker D8 ADVANCE) with a B

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3. RESULTS AND DISCUSSION The microstructures of CFx and d-CFx are characterized by SEM and TEM, the corresponding images are shown in Figure 1. As depicted in Figure 1a, CFx exhibits a typical layered stacking structure with large grain sizes in the range of several to 20 μm, which indicates that CFx was generated from the graphitelike precursor. The edge of CFx was slightly peeled off, which may result from the insertion of a large number of fluorine atoms (Figure 1a,b). The blurred edges of the pristine CFx particles (Figure 1a and Figure S1c) may correspond to their poor conductivity.21 It should be noted that the SEM image of d-CFx is much clear (Figure 1c and Figure S1a,b), revealing the better electron conductivity after the simple treatment. Furthermore, the morphology of d-CFx exhibits more cracks. These might be induced by being “struck” and “peeled off” by NMP molecules under 90 °C, which weakens the van der Waals-like forces between graphite fluoride layers. As shown in Figure 1d, the edges of d-CFx particles display fewer layers than does pristine CFx and present a disordered structure due to the damage by fluorine atoms and NMP molecules. However, the treatment is so mild that NMP molecules cannot intercalate into the interlayer and break the van der Waals-like forces between neighboring layers to form fluorographene. XRD patterns of CFx and as-prepared d-CFx are displayed in Figure 2a. Two broad peaks corresponding to the fluorinated

processes and the structural evolvement of d-CFx are illustrated in Scheme 1. From the optical photograph, only gray solids Scheme 1. Schematic Illustration of the Preparation of d-CFx

were observed when 10 mg of pristine CFx was sonicated in 5 mL of NMP. A homogeneous dispersion can be obtained when a similar amount of d-CFx is dispersed in 5 mL of NMP without ultrasonication. Furthermore, the d-CFx/NMP dispersion did not sediment after several months. The color of the d-CFx/ NMP suspension changed to black, indicating that the structure of d-CFx differed from that of CFx and the conductivity of dCFx increased, in line with the results of SEM. Maybe, assynthesized d-CFx possessed more electrons on the surface and the edge of d-CFx, which can form a pseudohydrogen bond with NMP (possessing a large closed conjugated system formed by free pz orbitals (π and π*).24 This also agrees with the hypotheses from the SEM of d-CFx of better electron conductivity. However, these treatments were not unusual enough to overcome the van der Waals attraction between two adjacent layers at intermediate temperature and atmosphere. As a result, amorphous and highly disordered CFx was synthesized without fluorographene,25 which was prepared by the solvothermal exfoliation of fluorinated graphite (F-graphite) through the intercalation of acetonitrile and chloroform with low boiling points. Galvanostatic cycling was performed to examine the electrochemical behavior of the d-CFx electrode with sodium. The first discharge−charge profile is presented in Figure 3a, and the data of the Na/CFx cell is also included for comparison. Compared to commercial CFx, a slightly increase in the discharge voltage and a remarkable decrease in the charge voltage is achieved in the Na battery with a d-CFx cathode. The average discharge voltage plateau of the d-CFx electrode is 2.17 V vs Na+/Na, which is about 80 mV higher than that of CFx, indicating that the d-CFx electrode can reduce the active energy and improve Na+ diffusion into interlayers of d-CFx and then reduce the diffusion path lengths for the sodium ion and facilitate the reaction between fluorine and the sodium ion during the discharging process significantly. The initial charge voltage plateau of the Na/d-CFx cell is at about 3.07 V, which is about 1.10 V lower than that of the Na/CFx cell (4.17 V), indicating enhanced electrochemical activity. Furthermore, the d-CFx electrode delivers a discharge capacity of 582.5 mA h/g at a current density of 200 mA/g in the first cycle, slightly higher than that of CFx, which may be attribute to the complete reaction between active fluorine and sodium.

Figure 2. (a) XRD pattern and (b) Raman spectrum of as-prapared dCFx, coupled with those of commercial CFx.

phase as shown in Figure 2a (black line) at around 12 and 40° are in good agreement with previous data.22,23 The d-CFx composite exhibits a featureless diffraction pattern occurring in an angular range between 10 and 35°, completely different from that of CFx power, suggestive of the formation of the extremely amorphous nature of d-CFx (red line in Figure 2a). The graphitic features of CFx and d-CFx were evidenced by Raman spectroscopy (Figure 2b). The Raman spectrum of CFx shows D and G bands at around 1350 and 1590 cm−1, respectively. The intensity of the D band of CFx was much higher than that of the G band, which may be attributed to the insertion of a large amount of fluorine into the layers of graphite, with a large number of the disordered carbon edge sites and low graphitization. By contrast, the D and G bands of d-CFx become extraordinary broad and overlapped, which indicates that the as-obtained d-CFx is disordered and plentiful defects (or disorders) on the surface have been introduced into the assynthesized d-CFx. As discussed above, as-prepared d-CF x exhibits an amorphous and highly disordered feature. The preparative C

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Figure 3. (a) Discharge/charge profiles for the Na/d-CFx and Na/CFx cells. (b) Cycle performance of Na/d-CFx cell at a current density of 200 mA/g (discharging to 1.5 V in 3 h), with the inset showing the discharge capacity as a function of cycle number for the Na/d-CFx cell.

Figure 4. (a) SEM image of a d-CFx electrode after the first discharging to 1.5 V; (b) ex situ XRD patterns of different stages of d-CFx electrodes in the pristine, discharged, and charged states; (c) TEM image; and (d) the corresponding SAED pattern of the d-CFx electrode after the first discharging to 1.5 V.

during the charging process.6 On the other hand, the existence of fluorine in the electrolyte may be another reason for the capacity fade of the cells as in lithium ion batteries.26 In addition, the cell deteriorated suddenly after 12 cycles, which may result from the destruction of the sodium anode, in line with dendrite formation in lithium batteries.27−29 The use of a simple sodium anode in an organic electrolyte resulted in an uneven growth of mossy Na, indicating unstable stripping and plating on the surface of the anode during cycles (Figure S2).30,31 These results encourage the further exploration of sodium/CFx batteries, with the cyclability of the sodium anode included. The rate capability of the d-CFx electrode is depicted in Figure S3. When the Na/d-CFx cell cycled at a much lower rate, the Coulombic efficiency is very low (∼60%) and the cycle stability is terrible. However, the cell exhibits a high voltage gap and a low discharge capacity at a higher rate than 0.33 C (the rate in the manuscript is 200 mA/g, discharging to 1.5 V in 3

Figure 3b shows the initial 12 cycles of the d-CFx electrode at a constant current density of 200 mA/g. The discharge capacity fades in the subsequent cycle gradually (see the inset), with no increase in voltage polarization after 12 cycles. The subsequent discharge plateau increases slightly to 2.26 V, together with the slight decrease in charge potential (about 3.00−3.04 V) corresponding to the lowest value (780 mV) reported in the state-of-the-art Na/CFx system.4−6 The d-CFx cathode exhibits much better electrochemical performance because of the enhanced rates of solvated Na+ diffusion and high charge mobility. However, the capacity faded gradually to 412.5 mAh/ g after 12 cycles, resembling the performance of Na/CFx and Na/(CFx/GNS/FeF3) cells.4,5 The gradual capacity decay should result from the loss of active fluorine during the cycles. Furthermore, the volume change and the degradation of a mixed conductive network would influence the reversibility of decomposed products, resembling the result in Na/CF0.75 batteries, in which the metastable structures do not reform D

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The Journal of Physical Chemistry C h), which could be responsible for the lower intrinsic electrical conductivity. Intensive research should be carried out to reveal this phenomenon and tackle the problem of poor rate performance. To investigate the electrochemical process of the Na/d-CFx cell, SEM, XRD, and TEM were used to examine the d-CFx electrodes in different stages, as shown in Figure 4. The surface of the pristine d-CFx electrode (seen in Figure S4a) is macroscopically smooth. When the Na/d-CFx cell was discharged to 1.5 V (Figure 4a), a copious number of nanoparticles can be seen on the surface of the d-CFx electrode, which was tentatively considered to be a discharge product of NaF. However, only a relevant low-resolution SEM image can be achieved because the discharge products were not stable and melted quickly under electron beam focusing. Furthermore, these nanoparticles disappeared after the charging process, indicating that discharge products NaF can be reversible decomposed (Figure S4b). The ex situ XRD measurements were used to detect the composition and structure of the sodiated and desodiated electrodes, as seen in Figure 4b. For all of the d-CFx electrodes, diffraction peaks located at 26.1, 38.2, 39.9, 52.8, 62.8, 70.4, and 76.0° (marked with asterisk) should be attributed to the titanium substrate (JCPDS 01-1197). The only broad peak between 10 and 35° of d-CFx was not observed in all electrodes in different stages, which may result from the relatively low intensity of amorphous d-CFx, in comparison with that of Ti. It is surprising that the diffraction patterns of different stages are similar, cubic crystal NaF with reflections at 38.7 and 56.0°, and are not detected after the discharging process. We made the hypothesis that the NaF discharge product is amorphous, different from crystal product NaF.4,5 To prove the above-mentioned assumption of amorphous discharge products NaF, ex situ TEM and SAED were carried out. Figure S5a shows an SAED pattern of the pristine d-CFx electrode, and the clear rings could be well assigned to the C phase (JCPDS 44-558).5 After the d-CFx electrode was discharged to 1.5 V, a significant number of particles were found to fill or were on the surface of the d-CFx electrode, which were estimated to be the discharge products (Figure 4c). Only a relevant low-resolution TEM image can be achieved because of the instability of the products. The SAED pattern of this region shows several rings (Figure 4d) instead of clear discrete diffraction spots of the NaF phase (JCPDS 36-1455); however, they were unambiguously different from those of the pristine d-CFx electrode (Figure S5a). These clear rings of the SAED patterns in Figure 4d were also assigned to the NaF phase (JCPDS 36-1455). In striking contrast, TEM images and SAED patterns of the d-CFx electrode after the charging process vary apparently from those of the discharge state (Figure S5b). The results evidence that the discharge products of the Na/d-CFx cell are amorphous, which agrees with the hypothesis of XRD patterns. To further validate the chemical composition of nanoparticles covering the surface of discharged d-CFx electrode, XPS measurements were conducted on a pristine d-CFx electrode and the discharged electrode, respectively. The XPS analysis (Figure 5) of the discharged d-CFx electrode reveals the presence of NaF. In the F 1s spectrum, the peak at 688.0 eV can be assigned to the C−F bond in d-CFx. The measured binding energy of 685.8 eV is in line with the earlier reports for NaF (685.7 eV).4,5 The absence of evidence for NaF in the XRD pattern and the presence of NaF in XPS imply that the

Figure 5. XPS analysis of the pristine d-CFx electrode and the discharged d-CFx electrode.

discharge products are amorphous NaF. As discussed above, the discharge products of the Na/d-CFx cell were amorphous, nanosized NaF. Here, we explored a simple and low-cost method to prepare d-CFx material for sodium secondary batteries, which exhibited the lowest polarization and capacity-decay rate of the Na/CFx batteries in previous work.4−6 On the basis of the results of SEM, TEM, XRD, and Raman spectra, as-prepared d-CFx was amorphous and highly disordered (Figure S6b), which facilitate Na+ or solvent Na+ diffusing into the interlayer and reacting with C−F bonds in d-CFx, leading to a higher discharge plateau and a much larger discharge capacity. Amorphous, nanosized discharge products of NaF filled or were on the surface of dCFx material, as evidenced by the results of XRD, TEM, SAED, and XPS (Figure S6d), and cubic NaF formed as a discharge product in Na batteries deploying commercial CFx, FMWCNTs, and the CGF composite as cathodes (Figure S6b, commercial CFx as the cathode, for example).4,5 The amorphous nanosize of NaF is of great significance in facilitating the deposition of NaF during the charging process, leading to a voltage gap of only 0.78 V. It is important to note that Na/d-CFx batteries have impressive rechargeability, which may be attributed to the fast decomposition of amorphous, nanosized NaF and the short diffusion paths of active fluorine to react with amorphous carbon, corresponding to a smaller loss of active fluorine and a lower capacity-decay rate as compared to that of current Na/CFx batteries.4−6

4. CONCLUSIONS Herein we have demonstrated that amorphous, highly disordered d-CFx, which can be synthesized via a facile synthesis route, was successfully utilized for sodium secondary batteries with high specific capacities, an optimal overpotential, and a lower capacity-decay rate. XRD, Raman spectra, and SAED evidence d-CFx being an amorphous, disordered material and fewer layers on the side for sodium batteries. During the discharging process, amorphous products of NaF with an extremely small size formed, which was indexed to play a decisive factor in minimizing the gap in the charge voltage. In some scenarios, the easier decomposition of NaF facilitates the rebonding of active F with the amorphous carbon framework, leading to a smaller loss of active fluorine and a slower capacitydecay rate. However, a stable sodium metal anode is another key issue that affects the life of batteries. Further detailed studies should be carried out to develop a sodium battery with E

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excellent energy efficiency, an ultralong cycle life, and a high energy density for practical applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b07126. SEM images of commercial CFx and d-CFx power at different magnifications; optical images of the sodium anode after several cycles; rate performance of Na/d-CFx batteries; SEM images of d-CFx electrodes; SAED pattern of a pristine d-CFx electrode; TEM of d-CFx electrode in charged state and the corresponding SAED pattern; proposed electrochemical reaction mechanism of Na/d-CFx and Na/CFx batteries; and the complete author list of reference 6 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86-21-24187673. Fax: +86-21-24188008. E-mail: jyxie@ mail.sim.ac.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (NSFC21373137). REFERENCES

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