English

• 0.   Introduction

  In recent years, secondary batteries have attracted much attention due to the application of electric vehicles and portable electronic device^[1-4]. However, with the development of military weapons, implantable medical devices, defence tools and aerospace devices, primary batteries with high energy density and high power density are indispensable^[5-8]. Among various primary batteries, Li/CF_x batteries are one of the promising owing to their high theoretical specific energy (2 180 Wh·kg^-1), high theoretical specific capacity (865 mAh·g^-1), wide operating temperature range (-40~170 ℃), high open circuit voltage (3.0~3.2 V), long storage life and good safety performance^[9-13]. Unfortunately, Li/CF_x battery has a strong polarization, low voltage platform and poor high rate performance in the process of discharge, which is mainly from the poor electronic conductivity of CF_x materials^[14-17]. These unfavourable factors have limited the application and development of Li/CF_x batteries in nowadays society.

  In order to solve these problems, many resear-chers have been carrying out different methods to improve the electronic conductivity of CF_x. The usual ways are covering with the conductive carbon layer or conductive polymer to enhance the electrochemical performance of CF_x materials. Zhang et al.^[18] obtained the carbon-coated CF_x by a heat treatment of a mixture of CF_x and polyvinyl difluoride (PVDF). The specific discharge capacity of carbon-coated CF_x was 370 mAh·g^-1 at 2C, while that of uncoated CF_x was 260 mAh·g^-1 at the same rate. The coating carbon improved the connectivity of particles and facilitated the construction of electronic channels, thus accelera-ting the diffusion of lithium ions and electron trans-port. Li et al.^[19] prepared the polyaniline-coated CF_x (x=1) (CF_x@PANI) with different thicknesses by using an in situ chemical oxidative polymerization method at 0 ℃. The CF_x@PANI had a discharge specific capacity of 843 mAh·g^-1 at 0.1C. In particular, the discharge rate of CF_x@PANI could reach up to 8C, which was much higher than that of pristine CF_x. The improved electrochemical performance of the coated CF_x materials came from the better conductivity of the polyaniline layer. Adding electrode materials with excellent conductivity such as MnO₂^[20], Ag₂V₄O₁₁^[21], LiV₃O₈^[22] and MoO₃^[23] to CF_x is a very effective method to improve the conductivity of cathode materials. For example, CF_x∥MnO₂ exhibited the energy density of 1 814 Wh·kg^-1 and the power density of 6 599 W·kg^-1 at 5C^[20], respectively. Another way to improve the electrochemical performance of CF_x materials is to prepare sub-fluorinated graphite. Sun et al. synthesized a fluorographene (FG) material by solvothermal exfoli-ation of F-graphite with acetonitrile and chloroform. The exfoliated FG showed a high power density of 4 038 W·kg^-1 at 3C, which was much higher than that of F-graphite^[24]. Other methods such as acid treatment, alkali treatment or ball milling also achieved better performance^[25-27].

  In this work, we prepared CF_x-Ru materials for the first time by a simple in-situ chemical modification. The introduction of Ru via the interaction between RuO₂ and CF₂ improved the conductivity of the material. Meanwhile, the reduction of inactive CF₂ on the surface of CF_x is beneficial to the diffusion of lithium ions. Compared with the pristine CF_x, CF_x-Ru has a great improvement in specific capacity, energy density and discharge potential. This simple method provides a new way to solve the low rate performance of carbon fluoride materials.

  1.   Experimental

  1.1   Synthesis of samples

  CF_x-Ru composites were prepared by a simple method of in situ modification. The preparation process was shown in Fig. 1a. Firstly, 0.3 g CF_x (x=0.71, Products fluorinated carbon, FLUOROCHEMICAL CO., LTD) was dispersed in mixture of water/ethanol (1:1, V:V, 30 mL) and stirred at room temperature for 30 min. 0.019 g RuCl₃·xH₂O was added into the CF_x suspens-ion. Then, 0.1 mol·L^-1 NaOH was dropped into the solution to keep the pH value at 8~9 and stirred at room temperature for 6 h. After the precipitation process, the mixture was filtered through a microporous membrane and washed thoroughly with deionized water and ethanol. Afterward, the mixture was dried overnight at 80 ℃. Finally, the mixture was treated at 450 ℃ under N₂ flow for 3 h to obtain the CF_x-Ru composites (Fig. 1a). For comparison, samples were also prepared through the same steps as above except in the addition of pristine CF_x, which was illustrated by Fig. 1b (The latter XRD results confirmed that it was RuO₂).

  Figure 1

  

  Figure 1.  Schematic illustration for preparation of CF_x-Ru (a) and RuO₂ (b) samples

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  1.2   Characterization

  The phases were characterized by X-ray diffrac-tion analysis (XRD, Bruker D8 Advance Diffractometer with Cu Kα radiation, λ=0.154 nm, target voltage was 40 kV, target current was 40 mA, scan speed was 0.5°·min^-1, step size was 0.02°, and intensity data were collected within 2θ range of 10°~80°). The morphology and composition of the samples were characterized by scanning electron microscope (SEM) and energy dispersive X-ray spectroscopic (EDX) analysis on a Hitachi S-3400N. The chemical structure and element valence of pristine CF_x and CF_x-Ru were measured by X-ray photoelectron spectroscopy (XPS, Kratos Analy-tical Ltd, Axis Ultra). Thermo gravimetric analysis (TGA) was performed using a TG/DTA6300 thermal analyzer between 25 and 500 ℃ with a heating rate of 5 ℃·min^-1 under N₂ flow. The specific surface area of samples was measured on a Brunauer-Emmett-Teller (BET, ASIQ COR 100-3).

  1.3   Electrochemical test

  Electrochemical properties of the samples were measured with CR2032 coin cells, which were assembled inside a NEWARE glove box (volume fraction of H₂O < 5×10^-7, volume fraction of O₂ < 5×10^-7). For discharge performance testing, metal lithium as the anode, whatman glass fibre filter as separator and LiPF₆ in EC-DMC (1:1, w/w) was used as electrolyte. The cathode electrode was synthesized through the following steps. The active material (80%(w/w)), the acetylene black (10%(w/w)) and the polyvinyl difluoride (PVDF, 10%(w/w)) were mixed and stirred in the N-methyl-2-pyrrolidone to form a uniform black slurry. The slurry was spread evenly on an Al foil and then dried at 80 ℃ for 12 h. The coated Al foil was cut into discs with a diameter of 14 mm and the discs were dried in vacuum at 120 ℃ for 24 h. Finally, the electrode was subjected to a constant current discharge test at a room temperature with a cut-off voltage of 1.5 V and different rates. The electrochemical impedance spectroscopy (EIS) data was obtained on an electrochemical working station (VMP3, Bio-Logic SA) at a frequency range of 100 kHz~10 MHz with 5 mV amplitude.

  2.   Results and discussion

  2.1   Material characterization

  Fig. 2a showed the XRD patterns of pristine CF_x and CF_x-Ru, respectively. It was evident that the three typical wide peaks corresponding to the CF_x phases at around 13.4°, 25.7° and 41.2°. The peak at around 25.7° could be assigned to the (002) reflection of graphite phases, which indicated a kind of layered type structure of pristine CF_x^[28-30]. The peak at about 2θ value of 13.4° could be indexed in a hexagonal system to the (001) reflection particularly for comp-ounds with higher fluorine content^{[28, 31-32]}. The peak at 41.2° could be assigned to the (100) reflection, which was related to the plane length of the C-C bond in the reticular system^[33-35]. The similar diffraction peaks at 2θ values of 13.4°, 25.7° and 41.2° had also appeared for CF_x-Ru and thus indicated that the incorporation of Ru didnt change the structure of the pristine CF_x. In addition, the diffraction peak of Ru could be clearly observed in the XRD pattern of CF_x-Ru. As shown in Fig. 2b, in the case where all other experi-mental conditions are the same except adding CF_x we got RuO₂ phase instead of elemental Ru. From this we inferred that CF_x reacted with the modified materials to form metallic Ru and the reaction mechanism will be further studied.

  Figure 2

  

  Figure 2.  XRD patterns of (a) CF_x, CF_x-Ru and (b) RuO₂

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  Fig. 3 showed the morphology images of the pristine CF_x and CF_x-Ru samples. As shown in Fig. 3a, the colour of the pristine CF_x was light gray, which indicated that the precursor has been highly fluorinated. After calcination with Ru-containning precursor the CF_x-Ru samples turned into bright black (Fig. 3c), which might relate with the decrease of F content in the materials. Fig. 3b showed the typical layered structure of pristine CF_x with particle sizes in the range of 1~10 μm. Compared to the pristine CF_x, the modified CF_x material (Fig. 3d) was still a layered stacked structure and showed a clearer image. In addition, the morphology of the modified CF_x exhibited more cracks like limestone, which would facilitate diffusion of lithium ion. However, Ru particles were not found in Fig. 3d because of the sizes of Ru particles were very tiny. The existence of Ru in the modified material was also confirmed by energy dispersive X-ray spectroscopy (EDS) mapping (Fig. 4) besides XRD results aforementioned. The C, F and Ru elements could be observed from the EDS elemental distribution map, further indicating that we had successfully prepared CF_x-Ru materials. TEM images shown in Fig. 5 gave a more intuitive distribution of Ru particles. It could be seen that the fine Ru nanoparticles were dispersed on the CF_x support. However, the red areas in the Fig. 5 showed that the Ru particles behaved with a slight agglomeration on the surface and edges of the CF_x material.

  Figure 3

  

  Figure 3.  Photos and SEM images of (a, b) pristine CF_x and (c, d) CF_x-Ru

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  Figure 4

  

  Figure 4.  EDS mappings of the modified sample CF_x-Ru

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  Figure 5

  

  Figure 5.  TEM images of CF_x-Ru

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  X-ray photoelectron spectroscopy (XPS), known as electron spectroscopy for chemical analysis (ESCA), was used for a semi-quantitative analysis technique of surface chemical properties of materials. From the above results we concluded that certain functional groups on the surface of CF_x appear to react with the modified material. In order to explored the reaction mechanism, we performed XPS analysis on the materials before and after modification. Fig. 6a showed the detailed elemental composition information acquired from the XPS survey spectra. Compared with the pristine CF_x sample, the modified material showed extra Ru signal besides C and F. From the Ru3d XPS spectrum in Fig. 6b, two peaks at 280.34 and 284.51 eV ascribed to Ru3d_5/2 and Ru3d_3/2 could be found, which meant that the Ru on the surface of modified material had the same chemical environment as elementary Ru^[36] while the binding energy of Ru3d_5/2 in RuO₂ was 281 eV^[37]. Fig. 6c and 6d showed that the F1s peaks were located at 688.53 and 688.41 eV, respectively for pristine CF_x and modified CF_x. The small shift towards lower binding energy for C-F revealed a change in the chemical environment of the F atoms, which reflected that the parts of the covalent C-F bonds were transformed into a semi-ionic bond after modification of Ru^[38]. Fig. 6e and 6f showed the C1s peaks of pristine and modified CF_x. The peaks around 286.5 and 284.80 eV were due to sp³ C-C bonds and sp² C=C bonds^[39-40] and peaks around 291.4 and 289.6 eV ascribed to C-F₂ bonds and C-F bonds, respectively^{[39, 41-42]}. The peak area ratio of C-F₂ to C-F for pristine CF_x was estimated at 0.4 while only 0.24 for CF_x-Ru, namely 40%(n/n) loss after modification, which meant reduction of C-F₂ inactive group. The ratios of F to C were calculated too, and the detailed results were shown in Table 1. It was very clear that the ratio of F to C of the modified CF_x was decreased compared with that of pristine CF_x (from 1.15 to 1).

  Figure 6

  

  Figure 6.  XPS spectra: (a) survey spectra, (b) Ru3d, (c, d) C1s and (e, f) F1s

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  Table 1

  

  Table 1.  Peak locations (eV) and assignments for CF_x and CF_x-Ru

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  Sample	Peak location of C1s / eV						Atomic fraction of element / %		nF/nC
  	C=C	C-C	C-F (Semi-ionic)	C-F (Covalent)	C-F2		C	F
  CFx	284.80	286.59	287.72	289.72	291.36		46.20	52.99	1.15
  CFx-Ru	284.80	286.54	287.81	289.64	291.45		49.13	49.32	1.00

  Fig. 7 showed the TGA curves of pristine CF_x and CF_x-Ru composites, which were performed in N₂ atmo-sphere with a heating rate of 5 ℃·min^-1. In addition, all samples were heated to 450 ℃ and kept in 450 ℃ for 3 h. As shown in Fig. 7, it can be obviously observed that the pristine CF_x exhibited a weight loss of 11.31%(w/w) at around 450 ℃, resulted from the evaporation of fluorocarbons during the thermal decomposition process, such as C₂F₄^[43-44]. Interestingly, the TGA curve of CF_x-Ru sample showed a weight loss of 17.84%(w/w) at around 450 ℃, which was higher than that of pristine CF_x. According to the literature reports^[45-47], Ru complexes, as a kind of effective defluorination reagents, could reduce the activation energy barrier of C-F bond and activate C-F bond. Combined with the XRD and XPS results, we concluded that RuO₂ reacted with CF₂ distributed on the edges or surfaces of CF_x during the calcination. This reaction consumed CF₂ and produced elemental germanium, which maked the modified material exhibited a lower fluorocarbon ratio and greater weight loss. This process could be described by Equation (1):

  Figure 7

  

  Figure 7.  TGA curves of CF_x and CF_x-Ru recorded at 5 ℃·min^-1 under nitrogen

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  $ {\rm{Ru}}{{\rm{O}}_2} + 2{{\rm{H}}_2}{\rm{O}} + 2{\rm{C}}{{\rm{F}}_2} \to {\rm{Ru}} + 2{\rm{C}}{{\rm{O}}_2} + 4{\rm{HF}} $

  (1)

  Generally, the surface structure and pore size distribution of materials had an important influence on the physicochemical properties, and the surface area and pore size might change during the modification. Therefore, The BET specific surface area and pore size distribution of all samples were studied by nitrogen adsorption techniques and the results were shown in Fig. 8 and Table 2. Fig. 8 showed the meso-porous characteristics of CF_x and CF_x-Ru. The pore size of the modified CF_x was slightly larger than that of pristine CF_x (Fig. 8a and 8c), which might be due to the cracks generated on the surface of the CF_x-Ru. This result was consistent with the images of SEM. The corresponding typical Ⅳ isotherms of before and after Ru modification were shown in Fig. 8b and 8d, and the specific surface areas of 257 and 353 m²·g^-1, respectively for the CF_x and CF_x-Ru, calculated from the isotherm were displayed in Table 2. The larger specific surface area could effectively improve the contact area between the electrode material and the electrolyte, which was favourable for the diffusion of lithium ions and increased the lithium storage capacity of the material.

  Figure 8

  

  Figure 8.  BJH pore size distribution of (a) pristine CF_x and (c) CF_x-Ru; N₂ adsorption-desorption isotherms of (b) pristine CF_x and (d) CF_x-Ru

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  Table 2

  

  Table 2.  Surface area of CF_x and CF_x-Ru

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  Sample	BET surface area / (m2·g-1)	Pore diameter / nm
  CFx	257	1.527
  CFx-Ru	353	1.707

  2.2   Electrochemical

  performance test The electrochemical performances of both pristine CF_x and CF_x-Ru composites electrodes were evaluated against Li in standard CR2032 type coin cells. Fig. 9a and 9b showed the galvanostatic discharge curves of the pristine CF_x and CF_x-Ru composites at different current rates from 0.05C to 5C. As shown in Fig. 9a, the specific capacity of pristine CF_x was 665 mAh·g^-1 at 0.05 C and 611 mAh g^-1 at 0.5C. Even at a 1C rate only a marginal attenuation was obviously seen in the discharge capacity and voltage platform. However, a drastic decrease happened at a 5C rate with a capacity of only 217 mAh·g^-1. Compared with the pristine material, CF_x-Ru composites exhibited higher rate capability and its discharge capacity was 693 mAh·g^-1 at 0.05C and 669 mAh·g^-1 at 0.5C, respectively. The specific capacity and voltage platform of the CF_x-Ru composites had been significantly improved compared to those of pristine CF_x. A capacity up to 605 mAh·g^-1 at a rate of 5C had been obtained. The excellent electrochemical performance of CF_x-Ru composites might benefit from the following aspects. First, the increase of semi-ionic C-F bonds and the deposition of Ru on the surface of pristine CF_x improved the conductivity of the cathode material during modification process. Secondly, CF₂ on the surface of CF_x was considered to be an inactive group which hindered the diffusion of lithium ions^[27]. The reduction of the CF₂ content was beneficial to solve the transmission disorder of lithium ions, thereby contributed to the improvement of the rate performance and capacity performance of the material. Thirdly, the larger specific surface of the CF_x-Ru composites could provide more electrochemically active sites, and thus resulted in better electrochemical performance.

  Figure 9

  

  Figure 9.  Galvanostatic discharge curves of (a) CF_x and (b) CF_x-Ru; (c) Average potential as a function of discharge rate and (d) the Ragone plot of the pristine CF_x and CF_x-Ru composites

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  Fig. 9c displayed the change of average discharge potential with C-rate both for the pristine CF_x and CF_x-Ru composites. It was obvious that the average potential of pristine CF_x declined drastically as increasing discharge rate, and its average potential was only 1.7 V at a discharge rate of 5C. Compared with pristine CF_x, the CF_x-Ru composites revealed a higher average potential of 2.5 V at 0.05C and of up to 2 V even at a rate of 5C. In order to depict the electrochemical performance of the pristine CF_x and modifed CF_x, the energy and power densities displayed in the Ragone plot (Fig. 9d). It could be observed that the energy density decreased with the increasing of power density. The calculated maximum energy densities were 1 575 and 1 697 Wh·kg^-1 for the pristine CF_x and modifed CF_x at 0.05C, respectively. Compared with the CF_x-Ru composites, the energy density decreased seriously with the increase of discharge rate for pristine CF_x due to the decline of the discharge capacity and average potential. The pristine CF_x showed energy and power densities were 371 Wh·kg^-1 and 7 418 W·kg^-1 at the rate of 5C, respectively, while the corresponding values of 1 197 Wh·kg^-1 and 8727 W·kg^-1 could be achieved for CF_x-Ru.

  EIS technology was applied to further investigate the effect of Ru modification on the electrochemical performance of CF_x materials. Fig. 10a showed the Nyquist plot, and the inset indicated the equivalent circuit used to fit the impedance data. As shown in Fig. 10a, the EIS curve of each sample was composed of a slanted straight line in the low frequency region and a depressed semicircle in the high frequency region. The semicircle represented the charge transfer resistance of lithium ions from the electrolyte to the surface of the cathode material of the battery system, expressed by R_ct; the low frequency linear impedance was attributed to the diffusion impedance of lithium ions in the cathode material, expressed by Z_w. The linear relationship between Z_re (Real impedance) and the square root of frequency (ω^-0.5) were shown in Fig. 10b, and the slope of this line was related to Warburg factor (σ_w). And also the lithium ion diffusion coefficients (D_Li) of CF_x and CF_x-Ru materials were calculated according to the commonly used equation^[48]. Detailed data were listed in Table 3. It could be seen from the Table 3 that the charge transfer resistance of CF_x-Ru composite (161 Ω) was lower than that of the original CF_x material (220 Ω). Meanwhile, the former showed a higher D_Li of 2.07×10^-11 than 4.68×10^-12 cm²·s^-1 for the latter, which could benefit the kinetic behaviour during discharging. These data could indicate that the metal Ru modification could improve the electron transfer ability and lithium ion diffusion ability of the material, thereby improved the discharge specific capacity of the electrode material and the discharge voltage platform.

  Figure 10

  

  Figure 10.  Impedance spectra of the CF_x and CF_x-Ru composites: (a) Nyquist plots and equivalent circuit; (b) Graph of Z_re plotted as a function of ω^-1/2 at low frequency

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  Table 3

  

  Table 3.  R_s, R_ct, σ_w and D_Li values of the two electrodes prepared with CF_x and CF_x-Ru

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  Electrode	Rs / Ω	Rct / Ω	σw / (Ω·cm2·s-0.5)	DLi / (cm2·s-1)
  CFx	45.73	220	56.55	4.68×10-12
  CFx-Ru	33.87	161	26.90	2.07×10-11

  3.   Conclusions

  In summary, we have successfully synthesized CF_x-Ru materials by a simple in-situ composite method. CF_x-Ru composites exhibited specific capacity of 605 mAh·g^-1, maximum power density of 8 727 W·kg^-1 and energy density of 1 197 Wh·kg^-1 at a current density of 5C, which are much higher than those of pristine CF_x. The reaction mechanism between CF_x materials and Ru-containing precursor has been thoroughly investigated by XRD, XPS, and TGA et al. The structure, morphology, specific surface area and pore size distribution of the materials have also been studied in detail to reveal the reasons for their good performance. The excellent electrochemical perfor-mance of CF_x-Ru materials may be explained by the reduction of n_F/n_C and the introduction of metal Ru via the process of modification, which improves the conductivity of the material. In addition, the larger specific surface area and pore size are beneficial to the Li⁺ diffusion and electron transfer, which signifi-cantly reduces the charge transfer impedance and improves the Li⁺ diffusion coefficients.

  
  
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  Figure 1  Schematic illustration for preparation of CF_x-Ru (a) and RuO₂ (b) samples

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  Figure 2  XRD patterns of (a) CF_x, CF_x-Ru and (b) RuO₂

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  Figure 3  Photos and SEM images of (a, b) pristine CF_x and (c, d) CF_x-Ru

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  Figure 4  EDS mappings of the modified sample CF_x-Ru

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  Figure 5  TEM images of CF_x-Ru

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  Figure 6  XPS spectra: (a) survey spectra, (b) Ru3d, (c, d) C1s and (e, f) F1s

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  Figure 7  TGA curves of CF_x and CF_x-Ru recorded at 5 ℃·min^-1 under nitrogen

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  Figure 8  BJH pore size distribution of (a) pristine CF_x and (c) CF_x-Ru; N₂ adsorption-desorption isotherms of (b) pristine CF_x and (d) CF_x-Ru

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  Figure 9  Galvanostatic discharge curves of (a) CF_x and (b) CF_x-Ru; (c) Average potential as a function of discharge rate and (d) the Ragone plot of the pristine CF_x and CF_x-Ru composites

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  Figure 10  Impedance spectra of the CF_x and CF_x-Ru composites: (a) Nyquist plots and equivalent circuit; (b) Graph of Z_re plotted as a function of ω^-1/2 at low frequency

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  Table 1.  Peak locations (eV) and assignments for CF_x and CF_x-Ru

  Sample	Peak location of C1s / eV						Atomic fraction of element / %		nF/nC
  	C=C	C-C	C-F (Semi-ionic)	C-F (Covalent)	C-F2		C	F
  CFx	284.80	286.59	287.72	289.72	291.36		46.20	52.99	1.15
  CFx-Ru	284.80	286.54	287.81	289.64	291.45		49.13	49.32	1.00

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  Table 2.  Surface area of CF_x and CF_x-Ru

  Sample	BET surface area / (m2·g-1)	Pore diameter / nm
  CFx	257	1.527
  CFx-Ru	353	1.707

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  Table 3.  R_s, R_ct, σ_w and D_Li values of the two electrodes prepared with CF_x and CF_x-Ru

  Electrode	Rs / Ω	Rct / Ω	σw / (Ω·cm2·s-0.5)	DLi / (cm2·s-1)
  CFx	45.73	220	56.55	4.68×10-12
  CFx-Ru	33.87	161	26.90	2.07×10-11

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