Journal of Materials Research and Technology Journal of Materials Research and Technology
Original Article
Synthesis of FeF2/carbon composite nanoparticle by one-pot solid state reaction as cathode material for lithium-ion battery
Mengyun Tanga, Zhengfu Zhanga,, , Zi Wanga, Jingfeng Liua, Hongge Yanb, Jinhui Pengc, Lei Xuc, Shenghui Guoc, Shaohua Juc, Guo Chenc
a Faculty of Materials Science and Engineering, Kunming University of Science and Technology, 650093 Kunming, China
b College of Materials Science and Engineering, Hunan University, Changsha 410082, China
c National Local Joint Engineering Laboratory of Engineering Applications of Microwave Energy and Equipment Technology, Kunming 650093, China
Received 15 January 2017, Accepted 25 May 2017
Abstract

FeF2/carbon composite nanoparticle was prepared by a one-pot thermal reaction using a mixture of ferrous oxalate and PTFE as precursor. FeF2 was obtained as the main phase according to the XRD patterns of the samples prepared in the present study. Furthermore, the FeF2 particle has a size of 50–100nm. Its electrochemical properties were studied in the 4.2–1.3V region at a current density of 60mAg−1. It exhibited an initial discharge capacity of 503.394mAhg−1 and still reserved discharge capacity of about 268.478, 211.34, 193.817 and 183.328mAhg−1 at the 2nd, 3rd, 10th, and 20th cycles, respectively.

Keywords
Lithium-ion battery, FeF2, Nanoparticles, Cathode material, Charge–discharge performance
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1Introduction

Lithium-ion batteries (LIBs) are important energy storage devices, and they are required with higher energy density for usage in electric/hybrid vehicle and grid scale storage [1]. As the commercial cathode material LiCoO2 or LiFePO4 has a capacity of 150 or 170mAhg−1, the key to improve the energy density of LIBs is looking for electrode materials with higher energy density [2]. Metal fluoride can store more than one lithium per molecula or per molecular unit a multi-electron conversion reaction, so its theoretical capacity can be very high. Besides, metal fluoride has a high voltage potential brought by high ionicity of M–F bond [3]. Iron fluoride is one of the most potential candidates used as cathode materials due to its high specific capacity, low cost, and low toxicity [4–6]. FeF2 has a capacity of 571mAhg−1 and thermodynamic reduction potential with Li is 2.66V [7–9]. So it is a good candidate for high energy density LIBs.

FeF2/carbon nanoparticles were prepared by a one-pot thermal reaction using a simple procedure based on the thermal decomposition and reaction of a mixture of ferrous oxalate and PTFE. The composite's electrochemical property as cathode electrode is tested.

2Material and methods2.1Experiment

The ferrous oxalate powder (FeC2O4) and PTFE ((C2F4)n) powder were utilized as experimental raw materials. First, ferrous oxalate and PTFE were mixed using mortar and pestle. The weight ratio of [FeC2O4]:[PTFE] was varied from 10:2.78 to 10:4.17, in which amount of PFTE are larger than that of a stoichiometric ratio. The PTFE is decomposed at low temperature between 400 and 600°C [10]. Then, the homogeneous mixture was heated in an Ar atmosphere at a rate of 8°C/min up to 500, 550 and 650°C, This temperature was kept for 1h and cooled to room temperature naturally. Finally, the product was collected and then subjected to structure and property characterization.

2.2Characterization

The crystal structure of the product was characterized by X-ray diffraction using Bruker D8 with Cu Kα radiation (λ=1.5406Å). Transmission electron microscopy (TEM) measurements were made on a Hitachi H-8100 electron microscope with an accelerating voltage of 200kV.

The electrodes were prepared by mixing active materials (80wt%), acetylene black (10wt%), and polyvinylidene fluoride (PVDF, 10wt%) in N-methyl-2-pyrrolidone (NMP). After the above slurries were uniformly spread onto an aluminum foil, the electrodes were dried at 100°C in vacuum for 24h. Then electrodes were pressed and cut into disks before transferring into an Argon-filled glove box. Coin cells (CR2025) were assembled using lithium metal as the counter electrode, Celgard 2400 membrane as the separator and LiPF6 (1M) in ethylene carbonate/dimethyl carbonate/ethylmethyl carbonate (EC/DMC/EMC, 1:1:1vol%) as the electrolyte. The cells were tested at current density of 60mAg−1 between 4.2V and 1.3V with a Neware battery testing system.

3Results and discussion

Fig. 1 shows the XRD patterns of FeF2 with a ratio of [FeC2O4]:[PTFE]=10:4.17 synthesized for 1h under 500, 550 and 650°C. It showed that the diffraction peaks corresponding to FeF2 observed as a main phase in the sample synthesized at 500, 550 and 650°C, although weak impurities corresponding to Fe2O3 and Fe3O4. However, the peak intensity of FeF2 was decreased with the increase in the calcination temperature. In addition, XRD patterns of the nanocomposites did not show C peaks, suggesting that the C sheets are amorphous. This can be attributed to the decomposition temperature of PTFE, in which a drastic weight loss leading to the decomposition of PTFE was confirmed over 400°C and PTFE is completely decomposed at 600°C [10].

Fig. 1.
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XRD patterns of FeF2 with a ratio of [FeC2O4]:[PTFE]=10:4.17 synthesized for 1h under 500, 550, and 650°C.

The electrochemical performance of the FeF2/carbon composite as cathode was evaluated at room temperature. Fig. 2 shows the initial charge and discharge curves of the FeF2 in the 4.2–1.3V region at a current density of 60mAg−1. The initial discharge capacity of the FeF2 at 500, 550 and 650°C can reach 212.67, 503.394 and 333.327mAhg−1, respectively. The reversible capacity of the FeF2 obtained in this study was lower than that of the theoretical specific capacity (571mAhg−1). These results may be due to the Fe2O3 and Fe3O4 impurity phases observed in the sample, which would decrease the discharge capacity. However, the reversible capacity of the obtained FeF2 was higher than that of the FeF2–carbon composite reported by Zhang et al. (345mAhg−1) [2]. The high discharge capacity is attributed to incorporation of carbon that might facilitate charge transfer at the interface. These results indicate that the optimum temperature is 550°C.

Fig. 2.
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Initial charge and discharge curves of FeF2 with a weight ratio of [FeC2O4]:[PTFE]=10:4.17 synthesized at 500, 550, and 650°C for 1h in the 4.2–1.3V region at a current density of 60mAg−1.

Fig. 3 shows the XRD patterns of the sample synthesized at 550°C for 1h with function of different weight ratio of [FeC2O4]:[PTFE]. It is obvious that the optimum weight ratio of [FeC2O4]:[PTFE] for synthesis of FeF2 as a single phase is 10:4.17. There is a weak impurity corresponding to Fe3O4 with different weight ratio of [FeC2O4]:[PTFE]. The peak intensity of FeF2 was increased and the impurity phase was decreased with the increase of PTFE. It is indicate that the optimum conditions of synthesis of FeF2 using PTFE as a fluorine source are flowing that; i) weight ratio of [FeC2O4]:[PTFE] is 10:4.17, ii) calcination temperature is 550°C.

Fig. 3.
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XRD patterns of FeF2 synthesized at 550°C for 1h with function of different weight ratio of [FeC2O4]:[PTFE].

Fig. 4a shows a TEM image of the synthesized FeF2/carbon nanoparticle. The image clearly shows that the obtained FeF2 sample has granular particle morphology and a particle size of 50–100nm. The carbon layer coated on the FeF2 shows thickness less than 20nm, providing a good electrical conduction path. Fig. 4b shows the clearly observable lattice fringes of the carbon layer, which correspond to an inter-planar (d) spacing of 0.5nm. This result suggests that the particle growth of FeF2 was effectively suppressed by using PTFE as the fluorine source due to the low decomposition temperature and high activation rate, which led to the short reaction time and low reaction temperature for synthesis of the FeF2 sample.

Fig. 4.
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TEM image of FeF2/carbon nanoparticle as with a weight ratio of [FeC2O4]:[PTFE]=10:4.17 synthesized at 550°C for 1h.

Fig. 5a shows the initial charge and discharge curves of the FeF2 in the 4.2–1.3V region. The initial discharge capacity of the FeF2 with a weight ratio of [FeC2O4]:[PTFE]=10:2.78, 10:3, 10:3.61, 10:4.17 can reach 136.032, 168.098, 303.835 and 503.394mAhg−1, respectively. It is due to the Fe3O4 impurity phase observed in the sample, which would decrease the discharge capacity. These results indicate that the optimum weight ratio of [FeC2O4]:[PTFE] is 10:4.17. Fig. 5b shows the charge–discharge curves as a weight ratio of [FeC2O4]:[PTFE]=10:4.17 at a current density of 60mAg−1. During the first cycle, the composite delivers a discharge capacity of 503.394mAhg−1, the reversible specific charge capacity is 300.427mAhg−1. The reversible capacity of charge and discharge also decreased with increasing cycle number. The discharge capacities are found to be 268.478, 211.34, 193.817 and 183.328mAhg−1 at the 2nd, 3rd, 10th, and 20th cycles, respectively. In addition, the voltage plateau was confirmed in the 2.0V in the discharge curves. This region involves the conversion reaction, accompanied by the formation of Fe and LiF. In the following cycles, the plateaus are almost the same, which may be attributed to the improvement on conductivity due to generation of iron nanoparticles in the conversion reaction. The voltage plateau is lower than the theoretical voltage (2.66V). The possible reasons are as follows. The thick carbon layer presents a longer diffusion length, thus limiting Li+ diffusion and hindering electrolyte penetration. So, decreasing the thickness of carbon layer and the particle size of FeF2 will be explored in further investigation.

Fig. 5.
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Initial charge and discharge curves of FeF2 synthesized at 550°C for 1h with function of different weight ratio of [FeC2O4]:[PTFE] in the 4.2–1.3V region at a current density of 60mAg−1 (a), and charge–discharge curves with a weight ratio of [FeC2O4]:[PTFE]=10:4.17 at a current density of 60mAg−1 (b).

4Conclusions

In summary, FeF2/carbon composite nanoparticle was synthesized successfully by a one-pot thermal reaction using polytetrafluoroethylene (PTFE) as a fluorine source. It is indicated that the optimum conditions of synthesis of a single phase FeF2 are flowing that: a) calcination temperature is 550°C; b) weight ratio of [FeC2O4]:[PTFE] is 10:4.17. The composite exhibits an initial discharge capacity of 503.394mAhg−1 at a current density of 60mAg−1 in voltage range of 4.2–1.3V. The specific discharge capacities to be 268.478, 211.34, 193.817 and 183.328mAhg−1 at the 2nd, 3rd, 10th, and 20th cycles, respectively.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

The authors gratefully acknowledge the Joint Funds of the National Natural Science Foundation of China (Grant No. U1202272).

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