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Preparation of anhydrous iron fluoride with porous fusiform structure and its application for Li-ion batteries
Microporous and Mesoporous Materials 253 (2017) 10e17
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Preparation of anhydrous iron fluoride with porous fusiform structure and its application for Li-ion batteries Hongxu Sun a, Haochen Zhou b, Zhanglin Xu a, Jing Ding a, Juan Yang a, *, Xiangyang Zhou a a b
School of Metallurgy and Environment, Central South University, Changsha 410083, China Department of Aerospace, Tsinghua University, Beijing, 100084, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 March 2017 Received in revised form 24 May 2017 Accepted 16 June 2017 Available online 19 June 2017
Iron fluorides are drawing the attention of researchers for their high specific capacity as cathode materials of lithium ion batteries. In this work, we focus on the structural adjustment of iron fluorides themselves to improve the electrochemical performances. A kind of porous FeF3 with a fusiform structure has been synthesized by a facile solvothermal method and subsequent heat treatment. The synthetic process of hydrated FeF3 was performed by using simple and economic raw materials (iron salt and HF in absolute ethyl alcohol). The crystal water was removed through a mild drying in air, and porosity is given through the slow dehydration process. Electrochemical performance shows that the as-synthesized porous FeF3 can deliver stable reversible capacities of 137.3 mA g1 for 100 cycles at 20 mA g1. Although the specific capacity is moderate among the current research results of iron fluorides, this open porous structure obviously enhance the cycling stability of iron fluorides, and it can be expected to be improved remarkably with further processing. © 2017 Elsevier Inc. All rights reserved.
Keywords: Cathode materials Iron fluoride Porous structure Heat treatment Electrochemical performance
1. Introduction Recently, FeF3, as a conversion type cathode material, has been developed as a promising cathode material candidate for lithium ion batteries, owing to its high capacity, well cycling stability, low cost and low toxicity [1e5]. Compared with the traditional ones such as LiFePO4, LiCoO2 [6e8], the different Li-storage mechanism makes FeF3 have a high theoretical specific capacity of 237 mAh g1 (1 e transfer) in the 2e4.5 V region, and a larger capacity of 712 mAh g1 (3 e transfer) in the 1.5e4.5 V region [9e12]. However, the high specific capacity is difficult to be fully utilized. The electronic insulation in nature prevents FeF3 from its practical application as a cathode electrode material. A large band gap induced by the high ionicity makes FeF3 exhibit insulating behavior, which gives rise to large voltage hysteresis, poor capacity retention and poor rate capability. Moreover, structural changes caused by conversion reaction process severely degrade the cyclability of FeF3 cathodes. Much effort had been taken to improve the electrochemical performance of FeF3, such as designing rational morphologies
* Corresponding author. E-mail address: [email protected] (J. Yang). http://dx.doi.org/10.1016/j.micromeso.2017.06.033 1387-1811/© 2017 Elsevier Inc. All rights reserved.
[1,12e14], adding doping element [15] and manufacturing composite with carbonaceous materials (carbon nanotubes, graphene and porous carbon et al.) [2,16,17]. The electro-activity of FeF3 was first enabled through the fabrication of carbon-metal fluoride nano-composites (CMFNCs) by using of high energy mechanical milling, the nanoparticles of FeF3 was encompassed in a matrix of carbon, which exhibited a high specific capacity of 600 mAh g1 at 70 C [3,4]. Recently, numerous new ideas were put forward. For example, FeF3 nanoflowers on CNT branches were fabricated through a solution method, the CNT branches provided a threedimensional electronic path and ensured the structural stability of the electrode during battery cycling [2]; Low-temperature in situ synthesis of FeF3 nano-crystals on reduced graphene sheets was proposed, the graphene sheets can prevent the agglomeration of FeF3 nano-crystals and improve the electrochemical properties of FeF3 [17]; Ma et al. prepared a hybrid nanostructure composed of three-dimensionally ordered macroporous FeF3 and an homogenous coating of poly (3, 4-ethylenedioxythiophene) (as a conductive agent) through a in situ template method, which provided a fast and continuous electron transport for electrochemical reaction [1]. These above-mentioned methods all enabled electrically insulating FeF3 to deliver a good electrochemical performance. But always, majorities of research focus on the fabrication of FeF3/C composite, it is far from enough for researchers to modify the
H. Sun et al. / Microporous and Mesoporous Materials 253 (2017) 10e17
structure and morphology of FeF3 itself to improve the cycling and rate performances [18]. At present, among anhydrous FeF3 and its hydrated forms (FeF3$0.33H2O, FeF3$0.5H2O and FeF3$3H2O), anhydrous FeF3 is investigated by most of the researchers due to its anhydrous feature which would not lead to side reaction between H2O and LiPF6 in electrolyte. Typically, the preparation of anhydrous FeF3 employs high temperature dehydration of FeF3$3H2O. However, different dehydration temperatures (150 Ce400 C typically) are employed in numerous reports [15e17], and there are few studies focusing on the impact of temperature on the structure and properties of FeF3. Herein, we synthesized a type of porous anhydrous FeF3 with a fusiform structure by a facile solvothermal method combined with subsequent moderate heat treatment, which was the first report as far as we know of removing the water of hydration at such low temperature (120 C) without any protective atmosphere. The assynthesized materials were characterized by a series of explicit detection (X-ray diffraction, scanning electron microscopy, highresolution transmission electron microscopy and nitrogen adsorption desorption). The results indicate that the fusiform FeF3 were composed of a mass of amorphous FeF3 nanospheres with an average diameter of 20 nm. A BET surface area of 70.25 m2 g1 and a BJH adsorption pore diameter at around 1e12 nm confirmed the existence of a micro-mesoporous hybrid structure. Scheme 1 shows the schematic of the formation mechanism of porous structure. Undoubtedly, the characteristic of porous structure will be beneficial for the electrochemical performances of FeF3 cathode materials. The electrochemical performance shows that an excellent reversible capacity of 137.3 mA g1 (with no decay) for 100 cycles in the 2e4.5 V region at 20 mA g1 is achieved. It also delivers a superior rate performance, a reversible capacity of 102.4 mAh g1 at 200 mA g1, 86.1 mAh g1 at 400 mA g1 and 60.0 mAh g1 even at 1000 mA g1 are retained. It should be noted that the rational structure design and the facile synthesis process can make FeF3 a promising cathode candidate of lithium rechargeable batteries. 2. Experimental 2.1. Preparation of materials FeF3 powders were synthesized by a facile solvothermal method. Firstly, 140 mL FeCl3 (0.02 M) ethanol solution was prepared, then, 60 mL HF solution (40%) was slowly added into FeCl3
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ethanol solution dropwise with stirring untill the mixed solution became colorless. After stirring for another 0.5 h, the mixed solution was transferred to a sealed Teflon-lined autoclave and heated in an oven at 60 C for 10 h. The pink precipitates were collected by centrifugation and washed with ethanol for several times, then dried at 80 C for 8 h. The pink FeF3$3H2O powders were obtained. The anhydrous light green FeF3 powders were obtained through a moderate heat treatment at 120 C for 72 h in an oven without any protective atmosphere. 2.2. Materials characterization The phases of the synthesized materials were characterized by transmission electron microscopy (TEM, JEM-2100F, Japan), scanning electron microscope (SEM, Nova NanoSEM230, American), and X-ray diffraction (XRD, Rigaku-TTRIII, Japan). The pore diameter distribution, total pore volume and specific surface area were identified through N2 adsorption-desorption isotherms, using a Surface Area and Porosity Analyzer (ASAP 2020HD88). 2.3. Electrode preparation and electrochemical measurements The iron fluoride cathode was made by mixing 10% acetylene black, 10% polyvinylidene fluoride (PVDF) and 80% active materials in N-methyl-2-pyrrolidinone (NMP). The active materials were fabricated by ball milling of as-synthesized FeF3 powders and acetylene black (85:15 wt %) for 2 h. The well mixed slurry was coated on aluminum foil to form a thin film and dried at 120 C overnight in a vacuum oven. The electrodes were cut into small pellets of 1.0 cm in diameter, and the coin cells were assembled with Li metal foil as the anode in an argonfilled glove box (Super 1220/750, Shanghai Mikrouna Co. Ltd.). A Celgard 2400 polypropylene membrane was served as the separator. LiPF6 (1 M) in ethylene carbonate (EC) and diethyl carbonate (DEC) with a volume ratio of 1: 1 as the electrolyte. The cells were tested between 2 V and 4.5 V (~vs. Li/Liþ) at different current densities of 20e1000 mA g1 on a LAND CT-2001A (Wuhan, China). Cyclic voltammograms (CV) were measured by an electrochemical workstation (PARSTAT 4000) at a scanning rate of 0.5 mV s1 in a voltage range of 4.5e2.0 V at room temperature. Also, electrochemical impedance spectra (EIS) were obtained by PARSTAT 4000 electrochemical workstation from 100 kHz to 10 mHz with a automatic scanning mode at room temperature.
Scheme 1. A schematic of the formation mechanism of porous structure.
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3. Results and discussions Usually, anhydrous FeF3 is unstable in air and will slowly absorb water molecules to form hydrated ones. And pure anhydrous FeF3 is difficult to achieve even under the protective atmosphere, because Fe2O3 is often detected while removing the water of crystallization from hydrated ones through the high temperature heat treatment. Therefore, in most cases, hydrated FeF3 are employed as cathode material, such as FeF3$0.33H2O and FeF3$0.5H2O [19e21]. The water molecules in FeF3 lattice have the effect of crystal structure stabilization. However, they are likely to bring about the side reaction with the electrolyte with the increase of charge/discharge cycles. As a conversion-type cathode material, the crystal structure of FeF3 is reconstructed constantly during cycling. The crystal water inside of FeF3 lattice is gradually exposed, so the water caused side reaction with the electrolyte will be intensified. The aggravated internal operating environment will affect the cycling performance of the battery. In this study, FeF3$3H2O is prepared through a facile solvothermal method at 60 C as precursor, and anhydrous FeF3 is obtained successfully from FeF3$3H2O through a mild heat treatment. As shown in Fig. 1a, the X-ray diffraction (XRD) patterns of the as-synthesized FeF3$3H2O and FeF3 powders are represented. All XRD peaks are identified as FeF3$3H2O (JCPDS 32e0464) and FeF3 (JCPDS 33e0647), respectively. FeF3 is produced from FeF3$3H2O through heat treatment of 120 C, and the relative low temperature induces widened XRD peaks of FeF3. Below 30 and around 55 in the XRD pattern of FeF3, the characteristic peaks of (220) and (262) can still be recognized, which may ascribe to the strong water-absorbing ability of anhydrous FeF3. A little of water in air can be absorbed by FeF3 during storing and detecting. Fig. 1c shows the unit cells (1 1 3) for FeF3$3H2O along the [001] direction and corresponding crystal structure variation after heat treatment. The structure of b-FeF3$3H2O can be thought of as two infinite chains of [FeF6]n and [FeF2(H2O)4]n as shown in the left side of Fig. 1c [22]. After heat treatment, the crystal water is removed, leading to an unstable structure. The middle and right side of Fig. 1c
shows one possible anhydrous structure of [FeF6]n and [FeF2]n and unit cells (1 1 3) for crystalline FeF3. Actually, because of the nearly identical radius of H2O and F in the crystal lattice, the crystal structure variation of FeF3$3H2O is more disordered than that shown in Fig. 1c. Fig. 2 shows the whole possible structural changes of single cell of FeF3$3H2O. The heat treatment temperature of 120 C is not high enough to make the FeF3 crystallize completely. So the structure without crystal water will recombine, which leads to an amorphous characteristic of the corresponding XRD peaks [23]. The constituent of heat treatment products of as-synthesized FeF3$3H2O at various temperatures (200e400 C) under nitrogen atmosphere is also characterized as shown in Fig. 1b. It is clearly presented from the XRD patterns that FeF3$0.33H2O and Fe2O3 are obtained at 200 C for 10 h and 400 C for 10 h, respectively. Anhydrous FeF3 is achieved at 300 C for 6 h, but the obvious characteristic diffraction peaks of FeF3$0.33H2O along the [110] and [220] direction are detected, as well as the characteristic diffraction peaks of Fe2O3 along the [110] and [113] direction. Thus it can be seen pure anhydrous iron fluoride is difficult to achieve, unless to prolong the heat treatment time at between 200 C and 300 C in a protective atmosphere [22,24]. It can be believed that anhydrous FeF3 can still be obtained at 120 C in nitrogen atmosphere with enough heat treatment time. By contrast, the dehydration approach performing by drying at 120 C in air is more facile and economical. In order to confirm the degree of dehydration and determine the heat treatment time, a weight variation of FeF3$3H2O under 120 C is detected in Fig. 3. The weight of anhydrous FeF3 covers 67.62% of the total weight of FeF3$3H2O and the FeF3$0.33H2O is 71.21%. After 2000 min, the residual weight is equal to the weight ratio of FeF3$0.33H2O. After about 3600 min, the residual weight tends to be constant, which is close to 67.62%. As shown in Fig. 3, we detected the weight variation curve of FeF3$3H2O under 120 C, which no researchers did this research at such low temperature at present, so an accurate heat treatment time could be ascertained to obtain the anhydrous FeF3. At 120 C, this temperature is
Fig. 1. X-ray diffraction (XRD) patterns for as-synthesized FeF3$3H2O and FeF3 powders (a) and the heat treatment products of as-synthesized FeF3$3H2O at various temperatures under nitrogen atmosphere (b); Unit cells (1 1 3) for FeF3$3H2O along the [001] direction and corresponding crystal structure variation after heat treatment (c).
H. Sun et al. / Microporous and Mesoporous Materials 253 (2017) 10e17
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Fig. 2. Single cell of FeF3$3H2O and its possible structural triple-group variation after dehydration.
Fig. 3. Weight variation curve of FeF3$3H2O with the prolongation of heat treatment time at 120 C with no protective atmosphere.
insufficient to oxidize FeF3 into Fe2O3, and the green color of the obtained anhydrous FeF3 also illustrates that no Fe2O3 exists in product. X-ray diffraction (XRD) patterns for different heat treatment time (12 h, 24 h, 48 h, 72h) of FeF3$3H2O are also given in Fig. S1. With the prolongation of heat treatment time, the diffraction peak of FeF3 at 23.8 increases gradually. These results suggest that the crystal water could be almost removed through this type of heat treatment and the amorphous characteristic of the XRD spectrum is caused by amorphous FeF3. The morphology of as-synthesized FeF3$3H2O and FeF3 powders was characterized by scanning electron microscopy (SEM). As shown in Fig. 4a and b, a regular fusiform morphologies of FeF3$3H2O and FeF3 are observed with an average length of 10e50 mm. After heat treatment, the internal structure of the particles has changed (Fig. 4b). It illustrates that Fe and F atoms (as
shown in Fig. 1b) are rearranged to form anhydrous FeF3 after the removal of crystal water. Interestingly, it can be found in Fig. 4c that a few of them agglomerate into a radiated aggregate. On the premise of other conditions keep unchanged, the influence of temperature on the morphology was investigated. Fig. S2a - c show the SEM images of FeF3$3H2O synthesized at different temperature (0 C, 70 C and 90 C). At other synthesis temperatures, fusiform morphology has not been observed, and the grain sizes of the cubical FeF3$3H2O precursor get bigger with the increase of the synthesis temperature. Thus it can be seen that the synthesis temperature is the key factor of controlling the products morphology. Fig. 4d shows the surface of the fusiform FeF3 particles, a mass of agglomerated primary nanoparticles can be observed. In order to have a better insight of the structure characteristics of the FeF3 particles, transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) were performed, the TEM image is shown in Fig. 5a. The primary nanoparticles with diameter of only about 20 nm can be seen clearly. Although the typical micro/nano structure is composed of homogeneous primary nanoparticles, it has relatively huge micro-sized grain and regular fusiform morphology. HRTEM image of the primary nanoparticles shows that their crystal structure is composed of amorphous region and crystalline region as shown in Fig. 5b. Fig. 5c represents the interplanar distance of FeF3 crystalline domain, 0.224 nm and 0.266 nm is in consistent with the distance between the lattice fringes in the typical (113) and (104) crystal plane of FeF3. However, there are many defects in the lattice structure, indicating the incomplete crystallization of FeF3. This phenomenon is also consistent with the XRD results. Coupled with the XRD results of FeF3 and FeF3$3H2O, it implies that the primary FeF3 nanoparticles is formed in the heat treatment stage. Obviously, the primary nanoparticles can improve the reaction activity of FeF3. The gaps between the primary nanoparticles are expected to
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Fig. 4. SEM image of the as-synthesized FeF3$3H2O powders (a); SEM images of the as-synthesized anhydrous FeF3 powders (bed).
Fig. 5. TEM image (a) and HRTEM image (bec) of as-synthesized FeF3 powders; Pore size distribution patterns and nitrogen adsorption desorption isotherms of FeF3 powders with the prolongation of heat treatment time at 120 C (d).
H. Sun et al. / Microporous and Mesoporous Materials 253 (2017) 10e17
increase the porosity of FeF3 particles, and the formation mechanism of porous structure is shown in Scheme 1. Water molecules inside the particles are removed slowly due to the relative low heat treatment temperature. After heat treatment, the crystalline structure of FeF3$3H2O recombines, and a mass of amorphous and incomplete crystalline regions are retained, as well as a large number of crystal water diffusion channels formed during the water removal process. This loose structure is beneficial to the formation of micro-porous structure and the transmission of Li ions during cycling. Nitrogen adsorption-desorption isotherms and pore size distribution patterns of the as-synthesized FeF3 powders with the prolongation of heat treatment time at 120 C are shown in Fig. 5d and e, the two nitrogen adsorption-desorption isotherms of products drying for 36 h and 72 h both depict typical type IV isotherms with mesoporous hysteresis loop indicating the existence of a mesoporous structure [16]. Also, it can be seen that more micropores generate with the prolongation of heat treatment time which illustrates that there are direct links between the generation of porous structure and the slow dehydration process. Key pore characteristics of FeF3$3H2O precursor, samples after drying at 120 C for 36 h and 72 h are listed in Table 1. The BET surface area of the FeF3 powders drying for 72 h is calculated to be 70.25 m2 g1 with a total pore volume of 0.21 cm3 g1, and 56.87 m2 g1 with a total pore volume of 0.20 cm3 g1 for the one drying for 36 h. The pore size distribution pattern of FeF3 powders drying for 72 h, fitted from the corresponding absorption curve by BJH mode, suggested that most pores are composed of micropores (around 1 nm) and mesopores (2 nme12 nm), and the average pore diameter is estimated to be 1.22 nm. The most pores of powders drying for 36 h are mainly composed of mesopores (more than 17 nm), and its average pore diameter is estimated to be 72.974 nm. The mesopores formed between the primary FeF3 nanoparticles and the micrpores formed in the FeF3 nanoparticles during 72 h heat treatment would be beneficial for the sufficient contact between electrolyte and FeF3 cathode material, and also buffer the volume effect in the charge/ discharge cycling. The electrochemical performances of as-synthesized FeF3 powders were tested. A Galvanostatic discharge-charge test was first performed in the 2e4.5 V region at 20 mA g1. As shown in Fig. 6a, a discharge capacity of 124.9 mAh g1 is achieved in the first cycle. With the increase of the cycle number, the discharge capacity has a rising trend. After 100 cycles, a high reversible specific capacity of 137.3 mAh g1 is achieved and no obvious capacity fading is observed. It is noteworthy that although the capacity is not very high compared with some existing research results, this performance is achieved only through structure adjustment of fluoride iron itself without any compositing or doping modification. In terms of the current research results, it is difficult for pure iron fluoride to exceed the capacity of 110 mAh g1 under the condition of cycling for 100 cycles [25,26]. Therefore, it can be determined that better performance would be achieved through further material modification. Fig. 6b shows the corresponding voltage-capacity curves of 1st, 10th, 20th, 50th and 100th cycle. It can be found that the electrochemical plateaus of charge and discharge process both raise
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gradually, indicating that the active substance is activated more sufficiently and the hysteresis voltage decreases with the cycling going on. There are two charge plateaus in the 1st cycle, but the plateau above 4.0 V disappears gradually after 100 cycles, that is to say, the voltage hysteresis weakens gradually. As shown in Fig. 6c, the rate performance was investigated, the charge and discharge capacities were measured at current densities varying from 20 to 1000 mA g1, it can deliver a reversible capacity of 102.4 mAh g1 at 200 mA g1, 86.1 mAh g1 at 400 mA g1 and 60.0 mAh g1 even at 1000 mA g1. Cyclic voltammograms measured from 1st to 15th cycle is shown in Fig. 6d. The cathodic peaks appear between 2.5 V and 3.0 V, and the anodic peaks appear near 3.75 V and 4.25 V. Generally, the active materials participate in the reversible reaction of Equ. 1 in the 2e4.5 V region. However, unlike the cathodic process, there are two anodic peaks in Liþ deintercalation process. This suggests that the reversible reaction has different reaction paths, and the second anodic peak near 4.25 V implies a large voltage hysteresis in the charging process, which may associate with the Liþ deintercalation process of “Li0.5FeF3” [27]. With the increase of the cycle number, the second anodic peak diminishes gradually, which suggests that the voltage hysteresis weakens. This result can also explain the variation trend of the voltage-capacity curves. This phenomenon may attribute to the porous structure of the assynthesized FeF3 powders. After pre-cycling for several cycles, the electrolyte permeates into the porous channels of the assynthesized porous FeF3 powders more sufficiently, which increases the reaction activity of active materials. The EIS data of as-synthesized FeF3 cathode fully charged after 1, 10, 20, 50, 100 cycles at 200 mA g1 are carried out in Fig. 6e. All the Nyquist plots are similar which consist of a single depressed semicircle in the high-to-medium frequency region relating to the charge-transfer between the electrolyte and electrode materials interface and an inclined line at low frequency relating to the Li ion diffusion in the electrode materials. Obviously, the charge-transfer resistance increase gradually with the increase of cycling numbers, which may relate to the generation of new interface in the process of the electrode reaction. However, after cycling for about 50 cycles, the charge-transfer resistance is almost invariant, and even the semicircle in 100th is smaller than that in 50th. It can be seen that the interface charge resistance and electrochemical reaction resistance tends towards stability with the cycling going on under high rate current density. Therefore, a superior cycling performance is achieved as shown in Fig. 6f, even at a high current density of 200 mA g1, a reversible capacity of 91.4 mAh g1 for 100 cycles is retained. In order to maintain the porous structure of the anhydrous FeF3, the electrochemical performances are tested in the region of 2e4.5 V, in which voltage range the storage energy reaction is mainly: Liþ þ e þ FeF3 ¼ LiFeF3, being regarded as a insertion/ extraction reaction like that of LiCoO2. Therefore, there will be no obvious changes for the porous structure of FeF3, and an excellent cycling is achieved as shown in Fig. 6a, f. For further verification, surfaces of the electrodes before cycling and after cycling for 50 cycles are checked by SEM in Fig. 7aec. It can be observed that the FeF3 particles on surface of electrodes have no obvious change even in discharged (Li inserted) state after 50 cycles in Fig. 7b. Fig. 7c
Table 1 Key pore characteristics of FeF3$3H2O precursor, samples after drying at 120 C for 36 h and 72 h. Samples
BET SSA (m2 g1)
Pore volume (cm3 g1)
Average pore diameter (nm)
FeF3$3H2O precursor After drying at 120 C for 36 h After drying at 120 C for 72 h
1.38 56.87 70.25
0.001 0.20 0.21
10.08 72.974 1.22
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Fig. 6. Electrochemical performances of as-synthesized FeF3 cathode measured at a voltage range of 2e4.5 V: cycling performance at a current rate of 20 mA g1 (a) and corresponding voltage-capacity curves of 1st, 10th, 20th, 50th and 100th cycle (b); Specific capacity at different current rates from 20 to 1000 mA g1 (c); Cyclic voltammograms from the 1st to 15th cycle measured at a scanning rate of 0.5 mV s1 (d); EIS data of as-synthesized FeF3 cathode fully charged after different cycles at 200 mA g1 (e); Cycling performance at a high current rate of 200 mA g1 (f).
Fig. 7. Surface of the electrodes before cycling (a) and after cycling (bec) for 50 cycles.
H. Sun et al. / Microporous and Mesoporous Materials 253 (2017) 10e17
shows that the structure of the active material on electrode recovers after 50 complete cycles, which guarantees the superior cycling performance of cathode material. 4. Conclusion Fusiform FeF3$3H2O has been synthesized successfully by a facile solvothermal method. Through a mild low temperature heat treatment, anhydrous porous FeF3 with partial amorphous characteristics was obtained. It indicates that moderate heat treatment of FeF3$3H2O can remove its crystal water and change its internal crystal structure to form nano-sized particles aggregate companied with the formation of porous structure. The micro-nano porous structure can improve the cycling performance of FeF3 cathode materials obviously. The electrochemical performances show that the porous structure feature enables the as-synthesized FeF3 cathode material delivers enhanced cycling performance (137.3 mA g1 at a current density of 20 mA g1 with no decay for 100 cycles) and rate performance (91.2 mA g1 at a current density of 200 mA g1 for 100 cycles). The satisfactory behaviors benefit from the high specific surface of the as-synthesized FeF3 powders and the accommodation for the volume effect of the porous structure. Therefore, to optimize rational morphology of FeF3 cathode materials should be paid more attention as an effective measure to improve their electrochemical performances. Acknowledgements This work was supported by the National Nature Science Foundation of China (no. 51204209 and 51274240), the Project of Innovation-driven Plan in Central South University and the Fundamental Research Funds for the Central Universities of Central South University (2017zzts122). Appendix A. Supplementary data Supplementary data related to this chapter can be found at http://dx.doi.org/10.1016/j.micromeso.2017.06.033. References [1] D. Ma, Z. Cao, H. Wang, X. Huang, L. Wang, X. Zhang, Three-dimensionally ordered macroporous FeF3 and its in situ homogenous polymerization coating for high energy and power density lithium ion batteries, Energy & Environ. Sci. 5 (2012) 8538e8542. [2] S.W. Kim, D.H. Seo, H. Gwon, J. Kim, K. Kang, Fabrication of FeF3 Nanoflowers on CNT branches and their application to high power lithium rechargeable batteries, Adv. Mater. 22 (2010) 5260e5264. [3] F. Badway, N. Pereira, F. Cosandey, G.G. Amatucci, Carbon-metal fluoride nanocomposites structure and electrochemistry of FeF3:C, J. Electrochem. Soc. 150 (2003) A1209eA1218. [4] F. Badway, F. Cosandey, N. Pereira, G.G. Amatucci, Carbon metal fluoride nanocomposites high-capacity reversible metal fluoride conversion materials as rechargeable positive electrodes for Li batteries, J. Electrochem. Soc. 150 (2003) A1318eA1327. [5] F. Cosandey, J.F. Alsharab, F. Badway, G.G. Amatucci, P. Stadelmann, EELS spectroscopy of iron fluorides and FeFx/C nanocomposite electrodes used in Li-ion batteries, Microsc. Microanal. Official J. Microsc. Soc. Am. Microbeam Analysis Soc. Microsc. Soc. Can. 13 (2007) 87e95.
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[6] K. Mizushima, P.C. Jones, P.J. Wiseman, J.B. Goodenough, LiCoO2 (0
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Preparation of anhydrous iron fluoride with porous fusiform structure and its application for Li-ion batteries Hongxu Sun a, Haochen Zhou b, Zhanglin Xu a, Jing Ding a, Juan Yang a, *, Xiangyang Zhou a a b
School of Metallurgy and Environment, Central South University, Changsha 410083, China Department of Aerospace, Tsinghua University, Beijing, 100084, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 March 2017 Received in revised form 24 May 2017 Accepted 16 June 2017 Available online 19 June 2017
Iron fluorides are drawing the attention of researchers for their high specific capacity as cathode materials of lithium ion batteries. In this work, we focus on the structural adjustment of iron fluorides themselves to improve the electrochemical performances. A kind of porous FeF3 with a fusiform structure has been synthesized by a facile solvothermal method and subsequent heat treatment. The synthetic process of hydrated FeF3 was performed by using simple and economic raw materials (iron salt and HF in absolute ethyl alcohol). The crystal water was removed through a mild drying in air, and porosity is given through the slow dehydration process. Electrochemical performance shows that the as-synthesized porous FeF3 can deliver stable reversible capacities of 137.3 mA g1 for 100 cycles at 20 mA g1. Although the specific capacity is moderate among the current research results of iron fluorides, this open porous structure obviously enhance the cycling stability of iron fluorides, and it can be expected to be improved remarkably with further processing. © 2017 Elsevier Inc. All rights reserved.
Keywords: Cathode materials Iron fluoride Porous structure Heat treatment Electrochemical performance
1. Introduction Recently, FeF3, as a conversion type cathode material, has been developed as a promising cathode material candidate for lithium ion batteries, owing to its high capacity, well cycling stability, low cost and low toxicity [1e5]. Compared with the traditional ones such as LiFePO4, LiCoO2 [6e8], the different Li-storage mechanism makes FeF3 have a high theoretical specific capacity of 237 mAh g1 (1 e transfer) in the 2e4.5 V region, and a larger capacity of 712 mAh g1 (3 e transfer) in the 1.5e4.5 V region [9e12]. However, the high specific capacity is difficult to be fully utilized. The electronic insulation in nature prevents FeF3 from its practical application as a cathode electrode material. A large band gap induced by the high ionicity makes FeF3 exhibit insulating behavior, which gives rise to large voltage hysteresis, poor capacity retention and poor rate capability. Moreover, structural changes caused by conversion reaction process severely degrade the cyclability of FeF3 cathodes. Much effort had been taken to improve the electrochemical performance of FeF3, such as designing rational morphologies
* Corresponding author. E-mail address: [email protected] (J. Yang). http://dx.doi.org/10.1016/j.micromeso.2017.06.033 1387-1811/© 2017 Elsevier Inc. All rights reserved.
[1,12e14], adding doping element [15] and manufacturing composite with carbonaceous materials (carbon nanotubes, graphene and porous carbon et al.) [2,16,17]. The electro-activity of FeF3 was first enabled through the fabrication of carbon-metal fluoride nano-composites (CMFNCs) by using of high energy mechanical milling, the nanoparticles of FeF3 was encompassed in a matrix of carbon, which exhibited a high specific capacity of 600 mAh g1 at 70 C [3,4]. Recently, numerous new ideas were put forward. For example, FeF3 nanoflowers on CNT branches were fabricated through a solution method, the CNT branches provided a threedimensional electronic path and ensured the structural stability of the electrode during battery cycling [2]; Low-temperature in situ synthesis of FeF3 nano-crystals on reduced graphene sheets was proposed, the graphene sheets can prevent the agglomeration of FeF3 nano-crystals and improve the electrochemical properties of FeF3 [17]; Ma et al. prepared a hybrid nanostructure composed of three-dimensionally ordered macroporous FeF3 and an homogenous coating of poly (3, 4-ethylenedioxythiophene) (as a conductive agent) through a in situ template method, which provided a fast and continuous electron transport for electrochemical reaction [1]. These above-mentioned methods all enabled electrically insulating FeF3 to deliver a good electrochemical performance. But always, majorities of research focus on the fabrication of FeF3/C composite, it is far from enough for researchers to modify the
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structure and morphology of FeF3 itself to improve the cycling and rate performances [18]. At present, among anhydrous FeF3 and its hydrated forms (FeF3$0.33H2O, FeF3$0.5H2O and FeF3$3H2O), anhydrous FeF3 is investigated by most of the researchers due to its anhydrous feature which would not lead to side reaction between H2O and LiPF6 in electrolyte. Typically, the preparation of anhydrous FeF3 employs high temperature dehydration of FeF3$3H2O. However, different dehydration temperatures (150 Ce400 C typically) are employed in numerous reports [15e17], and there are few studies focusing on the impact of temperature on the structure and properties of FeF3. Herein, we synthesized a type of porous anhydrous FeF3 with a fusiform structure by a facile solvothermal method combined with subsequent moderate heat treatment, which was the first report as far as we know of removing the water of hydration at such low temperature (120 C) without any protective atmosphere. The assynthesized materials were characterized by a series of explicit detection (X-ray diffraction, scanning electron microscopy, highresolution transmission electron microscopy and nitrogen adsorption desorption). The results indicate that the fusiform FeF3 were composed of a mass of amorphous FeF3 nanospheres with an average diameter of 20 nm. A BET surface area of 70.25 m2 g1 and a BJH adsorption pore diameter at around 1e12 nm confirmed the existence of a micro-mesoporous hybrid structure. Scheme 1 shows the schematic of the formation mechanism of porous structure. Undoubtedly, the characteristic of porous structure will be beneficial for the electrochemical performances of FeF3 cathode materials. The electrochemical performance shows that an excellent reversible capacity of 137.3 mA g1 (with no decay) for 100 cycles in the 2e4.5 V region at 20 mA g1 is achieved. It also delivers a superior rate performance, a reversible capacity of 102.4 mAh g1 at 200 mA g1, 86.1 mAh g1 at 400 mA g1 and 60.0 mAh g1 even at 1000 mA g1 are retained. It should be noted that the rational structure design and the facile synthesis process can make FeF3 a promising cathode candidate of lithium rechargeable batteries. 2. Experimental 2.1. Preparation of materials FeF3 powders were synthesized by a facile solvothermal method. Firstly, 140 mL FeCl3 (0.02 M) ethanol solution was prepared, then, 60 mL HF solution (40%) was slowly added into FeCl3
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ethanol solution dropwise with stirring untill the mixed solution became colorless. After stirring for another 0.5 h, the mixed solution was transferred to a sealed Teflon-lined autoclave and heated in an oven at 60 C for 10 h. The pink precipitates were collected by centrifugation and washed with ethanol for several times, then dried at 80 C for 8 h. The pink FeF3$3H2O powders were obtained. The anhydrous light green FeF3 powders were obtained through a moderate heat treatment at 120 C for 72 h in an oven without any protective atmosphere. 2.2. Materials characterization The phases of the synthesized materials were characterized by transmission electron microscopy (TEM, JEM-2100F, Japan), scanning electron microscope (SEM, Nova NanoSEM230, American), and X-ray diffraction (XRD, Rigaku-TTRIII, Japan). The pore diameter distribution, total pore volume and specific surface area were identified through N2 adsorption-desorption isotherms, using a Surface Area and Porosity Analyzer (ASAP 2020HD88). 2.3. Electrode preparation and electrochemical measurements The iron fluoride cathode was made by mixing 10% acetylene black, 10% polyvinylidene fluoride (PVDF) and 80% active materials in N-methyl-2-pyrrolidinone (NMP). The active materials were fabricated by ball milling of as-synthesized FeF3 powders and acetylene black (85:15 wt %) for 2 h. The well mixed slurry was coated on aluminum foil to form a thin film and dried at 120 C overnight in a vacuum oven. The electrodes were cut into small pellets of 1.0 cm in diameter, and the coin cells were assembled with Li metal foil as the anode in an argonfilled glove box (Super 1220/750, Shanghai Mikrouna Co. Ltd.). A Celgard 2400 polypropylene membrane was served as the separator. LiPF6 (1 M) in ethylene carbonate (EC) and diethyl carbonate (DEC) with a volume ratio of 1: 1 as the electrolyte. The cells were tested between 2 V and 4.5 V (~vs. Li/Liþ) at different current densities of 20e1000 mA g1 on a LAND CT-2001A (Wuhan, China). Cyclic voltammograms (CV) were measured by an electrochemical workstation (PARSTAT 4000) at a scanning rate of 0.5 mV s1 in a voltage range of 4.5e2.0 V at room temperature. Also, electrochemical impedance spectra (EIS) were obtained by PARSTAT 4000 electrochemical workstation from 100 kHz to 10 mHz with a automatic scanning mode at room temperature.
Scheme 1. A schematic of the formation mechanism of porous structure.
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3. Results and discussions Usually, anhydrous FeF3 is unstable in air and will slowly absorb water molecules to form hydrated ones. And pure anhydrous FeF3 is difficult to achieve even under the protective atmosphere, because Fe2O3 is often detected while removing the water of crystallization from hydrated ones through the high temperature heat treatment. Therefore, in most cases, hydrated FeF3 are employed as cathode material, such as FeF3$0.33H2O and FeF3$0.5H2O [19e21]. The water molecules in FeF3 lattice have the effect of crystal structure stabilization. However, they are likely to bring about the side reaction with the electrolyte with the increase of charge/discharge cycles. As a conversion-type cathode material, the crystal structure of FeF3 is reconstructed constantly during cycling. The crystal water inside of FeF3 lattice is gradually exposed, so the water caused side reaction with the electrolyte will be intensified. The aggravated internal operating environment will affect the cycling performance of the battery. In this study, FeF3$3H2O is prepared through a facile solvothermal method at 60 C as precursor, and anhydrous FeF3 is obtained successfully from FeF3$3H2O through a mild heat treatment. As shown in Fig. 1a, the X-ray diffraction (XRD) patterns of the as-synthesized FeF3$3H2O and FeF3 powders are represented. All XRD peaks are identified as FeF3$3H2O (JCPDS 32e0464) and FeF3 (JCPDS 33e0647), respectively. FeF3 is produced from FeF3$3H2O through heat treatment of 120 C, and the relative low temperature induces widened XRD peaks of FeF3. Below 30 and around 55 in the XRD pattern of FeF3, the characteristic peaks of (220) and (262) can still be recognized, which may ascribe to the strong water-absorbing ability of anhydrous FeF3. A little of water in air can be absorbed by FeF3 during storing and detecting. Fig. 1c shows the unit cells (1 1 3) for FeF3$3H2O along the [001] direction and corresponding crystal structure variation after heat treatment. The structure of b-FeF3$3H2O can be thought of as two infinite chains of [FeF6]n and [FeF2(H2O)4]n as shown in the left side of Fig. 1c [22]. After heat treatment, the crystal water is removed, leading to an unstable structure. The middle and right side of Fig. 1c
shows one possible anhydrous structure of [FeF6]n and [FeF2]n and unit cells (1 1 3) for crystalline FeF3. Actually, because of the nearly identical radius of H2O and F in the crystal lattice, the crystal structure variation of FeF3$3H2O is more disordered than that shown in Fig. 1c. Fig. 2 shows the whole possible structural changes of single cell of FeF3$3H2O. The heat treatment temperature of 120 C is not high enough to make the FeF3 crystallize completely. So the structure without crystal water will recombine, which leads to an amorphous characteristic of the corresponding XRD peaks [23]. The constituent of heat treatment products of as-synthesized FeF3$3H2O at various temperatures (200e400 C) under nitrogen atmosphere is also characterized as shown in Fig. 1b. It is clearly presented from the XRD patterns that FeF3$0.33H2O and Fe2O3 are obtained at 200 C for 10 h and 400 C for 10 h, respectively. Anhydrous FeF3 is achieved at 300 C for 6 h, but the obvious characteristic diffraction peaks of FeF3$0.33H2O along the [110] and [220] direction are detected, as well as the characteristic diffraction peaks of Fe2O3 along the [110] and [113] direction. Thus it can be seen pure anhydrous iron fluoride is difficult to achieve, unless to prolong the heat treatment time at between 200 C and 300 C in a protective atmosphere [22,24]. It can be believed that anhydrous FeF3 can still be obtained at 120 C in nitrogen atmosphere with enough heat treatment time. By contrast, the dehydration approach performing by drying at 120 C in air is more facile and economical. In order to confirm the degree of dehydration and determine the heat treatment time, a weight variation of FeF3$3H2O under 120 C is detected in Fig. 3. The weight of anhydrous FeF3 covers 67.62% of the total weight of FeF3$3H2O and the FeF3$0.33H2O is 71.21%. After 2000 min, the residual weight is equal to the weight ratio of FeF3$0.33H2O. After about 3600 min, the residual weight tends to be constant, which is close to 67.62%. As shown in Fig. 3, we detected the weight variation curve of FeF3$3H2O under 120 C, which no researchers did this research at such low temperature at present, so an accurate heat treatment time could be ascertained to obtain the anhydrous FeF3. At 120 C, this temperature is
Fig. 1. X-ray diffraction (XRD) patterns for as-synthesized FeF3$3H2O and FeF3 powders (a) and the heat treatment products of as-synthesized FeF3$3H2O at various temperatures under nitrogen atmosphere (b); Unit cells (1 1 3) for FeF3$3H2O along the [001] direction and corresponding crystal structure variation after heat treatment (c).
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Fig. 2. Single cell of FeF3$3H2O and its possible structural triple-group variation after dehydration.
Fig. 3. Weight variation curve of FeF3$3H2O with the prolongation of heat treatment time at 120 C with no protective atmosphere.
insufficient to oxidize FeF3 into Fe2O3, and the green color of the obtained anhydrous FeF3 also illustrates that no Fe2O3 exists in product. X-ray diffraction (XRD) patterns for different heat treatment time (12 h, 24 h, 48 h, 72h) of FeF3$3H2O are also given in Fig. S1. With the prolongation of heat treatment time, the diffraction peak of FeF3 at 23.8 increases gradually. These results suggest that the crystal water could be almost removed through this type of heat treatment and the amorphous characteristic of the XRD spectrum is caused by amorphous FeF3. The morphology of as-synthesized FeF3$3H2O and FeF3 powders was characterized by scanning electron microscopy (SEM). As shown in Fig. 4a and b, a regular fusiform morphologies of FeF3$3H2O and FeF3 are observed with an average length of 10e50 mm. After heat treatment, the internal structure of the particles has changed (Fig. 4b). It illustrates that Fe and F atoms (as
shown in Fig. 1b) are rearranged to form anhydrous FeF3 after the removal of crystal water. Interestingly, it can be found in Fig. 4c that a few of them agglomerate into a radiated aggregate. On the premise of other conditions keep unchanged, the influence of temperature on the morphology was investigated. Fig. S2a - c show the SEM images of FeF3$3H2O synthesized at different temperature (0 C, 70 C and 90 C). At other synthesis temperatures, fusiform morphology has not been observed, and the grain sizes of the cubical FeF3$3H2O precursor get bigger with the increase of the synthesis temperature. Thus it can be seen that the synthesis temperature is the key factor of controlling the products morphology. Fig. 4d shows the surface of the fusiform FeF3 particles, a mass of agglomerated primary nanoparticles can be observed. In order to have a better insight of the structure characteristics of the FeF3 particles, transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) were performed, the TEM image is shown in Fig. 5a. The primary nanoparticles with diameter of only about 20 nm can be seen clearly. Although the typical micro/nano structure is composed of homogeneous primary nanoparticles, it has relatively huge micro-sized grain and regular fusiform morphology. HRTEM image of the primary nanoparticles shows that their crystal structure is composed of amorphous region and crystalline region as shown in Fig. 5b. Fig. 5c represents the interplanar distance of FeF3 crystalline domain, 0.224 nm and 0.266 nm is in consistent with the distance between the lattice fringes in the typical (113) and (104) crystal plane of FeF3. However, there are many defects in the lattice structure, indicating the incomplete crystallization of FeF3. This phenomenon is also consistent with the XRD results. Coupled with the XRD results of FeF3 and FeF3$3H2O, it implies that the primary FeF3 nanoparticles is formed in the heat treatment stage. Obviously, the primary nanoparticles can improve the reaction activity of FeF3. The gaps between the primary nanoparticles are expected to
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Fig. 4. SEM image of the as-synthesized FeF3$3H2O powders (a); SEM images of the as-synthesized anhydrous FeF3 powders (bed).
Fig. 5. TEM image (a) and HRTEM image (bec) of as-synthesized FeF3 powders; Pore size distribution patterns and nitrogen adsorption desorption isotherms of FeF3 powders with the prolongation of heat treatment time at 120 C (d).
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increase the porosity of FeF3 particles, and the formation mechanism of porous structure is shown in Scheme 1. Water molecules inside the particles are removed slowly due to the relative low heat treatment temperature. After heat treatment, the crystalline structure of FeF3$3H2O recombines, and a mass of amorphous and incomplete crystalline regions are retained, as well as a large number of crystal water diffusion channels formed during the water removal process. This loose structure is beneficial to the formation of micro-porous structure and the transmission of Li ions during cycling. Nitrogen adsorption-desorption isotherms and pore size distribution patterns of the as-synthesized FeF3 powders with the prolongation of heat treatment time at 120 C are shown in Fig. 5d and e, the two nitrogen adsorption-desorption isotherms of products drying for 36 h and 72 h both depict typical type IV isotherms with mesoporous hysteresis loop indicating the existence of a mesoporous structure [16]. Also, it can be seen that more micropores generate with the prolongation of heat treatment time which illustrates that there are direct links between the generation of porous structure and the slow dehydration process. Key pore characteristics of FeF3$3H2O precursor, samples after drying at 120 C for 36 h and 72 h are listed in Table 1. The BET surface area of the FeF3 powders drying for 72 h is calculated to be 70.25 m2 g1 with a total pore volume of 0.21 cm3 g1, and 56.87 m2 g1 with a total pore volume of 0.20 cm3 g1 for the one drying for 36 h. The pore size distribution pattern of FeF3 powders drying for 72 h, fitted from the corresponding absorption curve by BJH mode, suggested that most pores are composed of micropores (around 1 nm) and mesopores (2 nme12 nm), and the average pore diameter is estimated to be 1.22 nm. The most pores of powders drying for 36 h are mainly composed of mesopores (more than 17 nm), and its average pore diameter is estimated to be 72.974 nm. The mesopores formed between the primary FeF3 nanoparticles and the micrpores formed in the FeF3 nanoparticles during 72 h heat treatment would be beneficial for the sufficient contact between electrolyte and FeF3 cathode material, and also buffer the volume effect in the charge/ discharge cycling. The electrochemical performances of as-synthesized FeF3 powders were tested. A Galvanostatic discharge-charge test was first performed in the 2e4.5 V region at 20 mA g1. As shown in Fig. 6a, a discharge capacity of 124.9 mAh g1 is achieved in the first cycle. With the increase of the cycle number, the discharge capacity has a rising trend. After 100 cycles, a high reversible specific capacity of 137.3 mAh g1 is achieved and no obvious capacity fading is observed. It is noteworthy that although the capacity is not very high compared with some existing research results, this performance is achieved only through structure adjustment of fluoride iron itself without any compositing or doping modification. In terms of the current research results, it is difficult for pure iron fluoride to exceed the capacity of 110 mAh g1 under the condition of cycling for 100 cycles [25,26]. Therefore, it can be determined that better performance would be achieved through further material modification. Fig. 6b shows the corresponding voltage-capacity curves of 1st, 10th, 20th, 50th and 100th cycle. It can be found that the electrochemical plateaus of charge and discharge process both raise
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gradually, indicating that the active substance is activated more sufficiently and the hysteresis voltage decreases with the cycling going on. There are two charge plateaus in the 1st cycle, but the plateau above 4.0 V disappears gradually after 100 cycles, that is to say, the voltage hysteresis weakens gradually. As shown in Fig. 6c, the rate performance was investigated, the charge and discharge capacities were measured at current densities varying from 20 to 1000 mA g1, it can deliver a reversible capacity of 102.4 mAh g1 at 200 mA g1, 86.1 mAh g1 at 400 mA g1 and 60.0 mAh g1 even at 1000 mA g1. Cyclic voltammograms measured from 1st to 15th cycle is shown in Fig. 6d. The cathodic peaks appear between 2.5 V and 3.0 V, and the anodic peaks appear near 3.75 V and 4.25 V. Generally, the active materials participate in the reversible reaction of Equ. 1 in the 2e4.5 V region. However, unlike the cathodic process, there are two anodic peaks in Liþ deintercalation process. This suggests that the reversible reaction has different reaction paths, and the second anodic peak near 4.25 V implies a large voltage hysteresis in the charging process, which may associate with the Liþ deintercalation process of “Li0.5FeF3” [27]. With the increase of the cycle number, the second anodic peak diminishes gradually, which suggests that the voltage hysteresis weakens. This result can also explain the variation trend of the voltage-capacity curves. This phenomenon may attribute to the porous structure of the assynthesized FeF3 powders. After pre-cycling for several cycles, the electrolyte permeates into the porous channels of the assynthesized porous FeF3 powders more sufficiently, which increases the reaction activity of active materials. The EIS data of as-synthesized FeF3 cathode fully charged after 1, 10, 20, 50, 100 cycles at 200 mA g1 are carried out in Fig. 6e. All the Nyquist plots are similar which consist of a single depressed semicircle in the high-to-medium frequency region relating to the charge-transfer between the electrolyte and electrode materials interface and an inclined line at low frequency relating to the Li ion diffusion in the electrode materials. Obviously, the charge-transfer resistance increase gradually with the increase of cycling numbers, which may relate to the generation of new interface in the process of the electrode reaction. However, after cycling for about 50 cycles, the charge-transfer resistance is almost invariant, and even the semicircle in 100th is smaller than that in 50th. It can be seen that the interface charge resistance and electrochemical reaction resistance tends towards stability with the cycling going on under high rate current density. Therefore, a superior cycling performance is achieved as shown in Fig. 6f, even at a high current density of 200 mA g1, a reversible capacity of 91.4 mAh g1 for 100 cycles is retained. In order to maintain the porous structure of the anhydrous FeF3, the electrochemical performances are tested in the region of 2e4.5 V, in which voltage range the storage energy reaction is mainly: Liþ þ e þ FeF3 ¼ LiFeF3, being regarded as a insertion/ extraction reaction like that of LiCoO2. Therefore, there will be no obvious changes for the porous structure of FeF3, and an excellent cycling is achieved as shown in Fig. 6a, f. For further verification, surfaces of the electrodes before cycling and after cycling for 50 cycles are checked by SEM in Fig. 7aec. It can be observed that the FeF3 particles on surface of electrodes have no obvious change even in discharged (Li inserted) state after 50 cycles in Fig. 7b. Fig. 7c
Table 1 Key pore characteristics of FeF3$3H2O precursor, samples after drying at 120 C for 36 h and 72 h. Samples
BET SSA (m2 g1)
Pore volume (cm3 g1)
Average pore diameter (nm)
FeF3$3H2O precursor After drying at 120 C for 36 h After drying at 120 C for 72 h
1.38 56.87 70.25
0.001 0.20 0.21
10.08 72.974 1.22
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Fig. 6. Electrochemical performances of as-synthesized FeF3 cathode measured at a voltage range of 2e4.5 V: cycling performance at a current rate of 20 mA g1 (a) and corresponding voltage-capacity curves of 1st, 10th, 20th, 50th and 100th cycle (b); Specific capacity at different current rates from 20 to 1000 mA g1 (c); Cyclic voltammograms from the 1st to 15th cycle measured at a scanning rate of 0.5 mV s1 (d); EIS data of as-synthesized FeF3 cathode fully charged after different cycles at 200 mA g1 (e); Cycling performance at a high current rate of 200 mA g1 (f).
Fig. 7. Surface of the electrodes before cycling (a) and after cycling (bec) for 50 cycles.
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shows that the structure of the active material on electrode recovers after 50 complete cycles, which guarantees the superior cycling performance of cathode material. 4. Conclusion Fusiform FeF3$3H2O has been synthesized successfully by a facile solvothermal method. Through a mild low temperature heat treatment, anhydrous porous FeF3 with partial amorphous characteristics was obtained. It indicates that moderate heat treatment of FeF3$3H2O can remove its crystal water and change its internal crystal structure to form nano-sized particles aggregate companied with the formation of porous structure. The micro-nano porous structure can improve the cycling performance of FeF3 cathode materials obviously. The electrochemical performances show that the porous structure feature enables the as-synthesized FeF3 cathode material delivers enhanced cycling performance (137.3 mA g1 at a current density of 20 mA g1 with no decay for 100 cycles) and rate performance (91.2 mA g1 at a current density of 200 mA g1 for 100 cycles). The satisfactory behaviors benefit from the high specific surface of the as-synthesized FeF3 powders and the accommodation for the volume effect of the porous structure. Therefore, to optimize rational morphology of FeF3 cathode materials should be paid more attention as an effective measure to improve their electrochemical performances. Acknowledgements This work was supported by the National Nature Science Foundation of China (no. 51204209 and 51274240), the Project of Innovation-driven Plan in Central South University and the Fundamental Research Funds for the Central Universities of Central South University (2017zzts122). Appendix A. Supplementary data Supplementary data related to this chapter can be found at http://dx.doi.org/10.1016/j.micromeso.2017.06.033. References [1] D. Ma, Z. Cao, H. Wang, X. Huang, L. Wang, X. Zhang, Three-dimensionally ordered macroporous FeF3 and its in situ homogenous polymerization coating for high energy and power density lithium ion batteries, Energy & Environ. Sci. 5 (2012) 8538e8542. [2] S.W. Kim, D.H. Seo, H. Gwon, J. Kim, K. Kang, Fabrication of FeF3 Nanoflowers on CNT branches and their application to high power lithium rechargeable batteries, Adv. Mater. 22 (2010) 5260e5264. [3] F. Badway, N. Pereira, F. Cosandey, G.G. Amatucci, Carbon-metal fluoride nanocomposites structure and electrochemistry of FeF3:C, J. Electrochem. Soc. 150 (2003) A1209eA1218. [4] F. Badway, F. Cosandey, N. Pereira, G.G. Amatucci, Carbon metal fluoride nanocomposites high-capacity reversible metal fluoride conversion materials as rechargeable positive electrodes for Li batteries, J. Electrochem. Soc. 150 (2003) A1318eA1327. [5] F. Cosandey, J.F. Alsharab, F. Badway, G.G. Amatucci, P. Stadelmann, EELS spectroscopy of iron fluorides and FeFx/C nanocomposite electrodes used in Li-ion batteries, Microsc. Microanal. Official J. Microsc. Soc. Am. Microbeam Analysis Soc. Microsc. Soc. Can. 13 (2007) 87e95.
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[6] K. Mizushima, P.C. Jones, P.J. Wiseman, J.B. Goodenough, LiCoO2 (0